I got a little further by adding some bootargs like this:
setenv bootargs ubi.mtd=rootfs
And the kernel logs look more promising.. It looks more like it can't find a UBIFS driver? I'll try modifying the partitions in the DTS as well..
[ 0.000000] Booting Linux on physical CPU 0x0
[ 0.000000] Linux version 4.9.86 (tim@ITDept002) (gcc version 7.3.0 (OpenWrt GCC 7.3.0 r6473-63dc6ba) ) #0 SMP Wed Mar 14 09:20:28
[ 0.000000] CPU: ARMv7 Processor [410fc075] revision 5 (ARMv7), cr=10c5387d
[ 0.000000] CPU: div instructions available: patching division code
[ 0.000000] CPU: PIPT / VIPT nonaliasing data cache, VIPT aliasing instruction cache
[ 0.000000] OF: fdt:Machine model: Netgear WAC510
[ 0.000000] Memory policy: Data cache writealloc
[ 0.000000] percpu: Embedded 13 pages/cpu @cfda9000 s20940 r8192 d24116 u53248
[ 0.000000] Built 1 zonelists in Zone order, mobility grouping on. Total pages: 63744
[ 0.000000] Kernel command line: ubi.mtd=rootfs root=ubiblock0_2
[ 0.000000] Bootloader command line not present
[ 0.000000] PID hash table entries: 1024 (order: 0, 4096 bytes)
[ 0.000000] Dentry cache hash table entries: 32768 (order: 5, 131072 bytes)
[ 0.000000] Inode-cache hash table entries: 16384 (order: 4, 65536 bytes)
[ 0.000000] Memory: 248456K/257024K available (3839K kernel code, 246K rwdata, 1240K rodata, 244K init, 242K bss, 8568K reserved,)
[ 0.000000] Virtual kernel memory layout:
[ 0.000000] vector : 0xffff0000 - 0xffff1000 ( 4 kB)
[ 0.000000] fixmap : 0xffc00000 - 0xfff00000 (3072 kB)
[ 0.000000] vmalloc : 0xd0800000 - 0xff800000 ( 752 MB)
[ 0.000000] lowmem : 0xc0000000 - 0xd0000000 ( 256 MB)
[ 0.000000] pkmap : 0xbfe00000 - 0xc0000000 ( 2 MB)
[ 0.000000] modules : 0xbf000000 - 0xbfe00000 ( 14 MB)
[ 0.000000] .text : 0xc0208000 - 0xc05c7f78 (3840 kB)
[ 0.000000] .init : 0xc071f000 - 0xc075c000 ( 244 kB)
[ 0.000000] .data : 0xc075c000 - 0xc0799a40 ( 247 kB)
[ 0.000000] .bss : 0xc079b000 - 0xc07d78e8 ( 243 kB)
[ 0.000000] SLUB: HWalign=64, Order=0-3, MinObjects=0, CPUs=4, Nodes=1
[ 0.000000] Hierarchical RCU implementation.
[ 0.000000] NR_IRQS:16 nr_irqs:16 16
[ 0.000000] arm_arch_timer: Architected cp15 timer(s) running at 48.00MHz (virt).
[ 0.000000] clocksource: arch_sys_counter: mask: 0xffffffffffffff max_cycles: 0xb11fd3bfb, max_idle_ns: 440795203732 ns
[ 0.000007] sched_clock: 56 bits at 48MHz, resolution 20ns, wraps every 4398046511096ns
[ 0.000021] Switching to timer-based delay loop, resolution 20ns
[ 0.000403] Calibrating delay loop (skipped), value calculated using timer frequency.. 96.00 BogoMIPS (lpj=480000)
[ 0.000421] pid_max: default: 32768 minimum: 301
[ 0.000520] Mount-cache hash table entries: 1024 (order: 0, 4096 bytes)
[ 0.000532] Mountpoint-cache hash table entries: 1024 (order: 0, 4096 bytes)
[ 0.001117] CPU: Testing write buffer coherency: ok
[ 0.001427] Setting up static identity map for 0x80208280 - 0x802082d8
[ 0.005402] Brought up 4 CPUs
[ 0.005421] SMP: Total of 4 processors activated (384.00 BogoMIPS).
[ 0.005429] CPU: All CPU(s) started in SVC mode.
[ 0.017400] VFP support v0.3: implementor 41 architecture 2 part 30 variant 7 rev 5
[ 0.017595] clocksource: jiffies: mask: 0xffffffff max_cycles: 0xffffffff, max_idle_ns: 19112604462750000 ns
[ 0.017624] futex hash table entries: 1024 (order: 4, 65536 bytes)
[ 0.017786] pinctrl core: initialized pinctrl subsystem
[ 0.019077] NET: Registered protocol family 16
[ 0.019352] DMA: preallocated 256 KiB pool for atomic coherent allocations
[ 0.049985] cpuidle: using governor ladder
[ 0.080013] cpuidle: using governor menu
[ 0.120897] msm_bus_fabric_init_driver
[ 0.121165] msm_bus_device 580000.ad-hoc-bus: Util-fact is missing, default to 100
[ 0.121181] msm_bus_device 580000.ad-hoc-bus: Vrail-comp is missing, default to 100
[ 0.121200] msm_bus_device 580000.ad-hoc-bus: Failed to get bus clk for bus4096 ctx1
[ 0.121241] msm_bus_device 580000.ad-hoc-bus: Util-fact is missing, default to 100
[ 0.121254] msm_bus_device 580000.ad-hoc-bus: Vrail-comp is missing, default to 100
[ 0.121269] msm_bus_device 580000.ad-hoc-bus: Failed to get bus clk for bus1024 ctx1
[ 0.180041] usbcore: registered new interface driver usbfs
[ 0.180124] usbcore: registered new interface driver hub
[ 0.180238] usbcore: registered new device driver usb
[ 0.180341] pps_core: LinuxPPS API ver. 1 registered
[ 0.180352] pps_core: Software ver. 5.3.6 - Copyright 2005-2007 Rodolfo Giometti <giometti@linux.it>
[ 0.180380] PTP clock support registered
[ 0.181655] clocksource: Switched to clocksource arch_sys_counter
[ 0.182663] NET: Registered protocol family 2
[ 0.183297] TCP established hash table entries: 2048 (order: 1, 8192 bytes)
[ 0.183344] TCP bind hash table entries: 2048 (order: 2, 16384 bytes)
[ 0.183392] TCP: Hash tables configured (established 2048 bind 2048)
[ 0.183458] UDP hash table entries: 256 (order: 1, 8192 bytes)
[ 0.183491] UDP-Lite hash table entries: 256 (order: 1, 8192 bytes)
[ 0.183677] NET: Registered protocol family 1
[ 0.184927] No memory allocated for crashlog
[ 0.185167] workingset: timestamp_bits=30 max_order=16 bucket_order=0
[ 0.191140] squashfs: version 4.0 (2009/01/31) Phillip Lougher
[ 0.191155] jffs2: version 2.2 (NAND) (SUMMARY) (LZMA) (RTIME) (CMODE_PRIORITY) (c) 2001-2006 Red Hat, Inc.
[ 0.195650] io scheduler noop registered
[ 0.195666] io scheduler deadline registered (default)
[ 0.199771] tcsr 194b000.tcsr: setting usb hs phy mode select = e700e7
[ 0.199840] tcsr 1953000.ess_tcsr: setting ess interface select = 0
[ 0.199894] tcsr 1949000.tcsr: setting wifi_glb_cfg = 41000000
[ 0.199944] tcsr 1957000.tcsr: setting wifi_noc_memtype_m0_m2 = 2222222
[ 0.200092] Serial: 8250/16550 driver, 2 ports, IRQ sharing disabled
[ 0.200853] msm_serial 78af000.serial: msm_serial: detected port #0
[ 0.200898] msm_serial 78af000.serial: uartclk = 1843200
[ 0.200944] 78af000.serial: ttyMSM0 at MMIO 0x78af000 (irq = 125, base_baud = 115200) is a MSM
[ 0.200969] msm_serial: console setup on port #0
[ 0.732417] console [ttyMSM0] enabled
[ 0.737445] msm_serial: driver initialized
[ 0.747900] loop: module loaded
[ 0.749022] spi_qup 78b5000.spi: IN:block:16, fifo:64, OUT:block:16, fifo:64
[ 0.750731] m25p80 spi0.0: found s25fl016k, expected mx25l1606e
[ 0.762698] random: fast init done
[ 0.766125] m25p80 spi0.0: s25fl016k (2048 Kbytes)
[ 0.766297] 9 ofpart partitions found on MTD device spi0.0
[ 0.771031] Creating 9 MTD partitions on "spi0.0":
[ 0.776526] 0x000000000000-0x000000040000 : "0:SBL1"
[ 0.782298] 0x000000040000-0x000000060000 : "0:MIBIB"
[ 0.787325] 0x000000060000-0x0000000c0000 : "0:QSEE"
[ 0.792282] 0x0000000c0000-0x0000000d0000 : "0:CDT"
[ 0.797198] 0x0000000d0000-0x0000000e0000 : "0:DDRPARAMS"
[ 0.801879] 0x0000000e0000-0x0000000f0000 : "0:APPSBLENV"
[ 0.807387] 0x0000000f0000-0x0000001e0000 : "0:APPSBL"
[ 0.812826] 0x0000001e0000-0x0000001f0000 : "0:MANUDATA"
[ 0.817870] 0x0000001f0000-0x000000200000 : "0:ART"
[ 0.824329] libphy: ipq40xx_mdio: probed
[ 0.861100] ESS reset ok!
[ 0.894098] ESS reset ok!
[ 1.322165] libphy: Fixed MDIO Bus: probed
[ 1.322190] libphy: mdio_driver_register: qca8k
[ 1.422378] i2c /dev entries driver
[ 1.424251] cpufreq: cpufreq_online: CPU0: Running at unlisted freq: 666000 KHz
[ 1.424741] cpufreq: cpufreq_online: CPU0: Unlisted initial frequency changed to: 716800 KHz
[ 1.432516] sdhci: Secure Digital Host Controller Interface driver
[ 1.440646] sdhci: Copyright(c) Pierre Ossman
[ 1.447030] sdhci-pltfm: SDHCI platform and OF driver helper
[ 1.503699] nand: device found, Manufacturer ID: 0xef, Chip ID: 0xaa
[ 1.503933] nand: Winbond W25N01GV 1G 3.3V 8-bit
[ 1.509356] nand: 128 MiB, SLC, erase size: 128 KiB, page size: 2048, OOB size: 64
[ 1.514414] Scanning device for bad blocks
[ 1.932425] random: crng init done
[ 2.036913] 1 ofpart partitions found on MTD device spi0.1
[ 2.036938] Creating 1 MTD partitions on "spi0.1":
[ 2.041293] 0x000000000000-0x000003800000 : "rootfs"
[ 2.047385] mtd: device 9 (rootfs) set to be root filesystem
[ 2.053425] mtdsplit: no squashfs found in "rootfs"
[ 2.057883] NET: Registered protocol family 10
[ 2.062867] NET: Registered protocol family 17
[ 2.066148] 8021q: 802.1Q VLAN Support v1.8
[ 2.070430] Registering SWP/SWPB emulation handler
[ 2.078314] ubi0: attaching mtd9
[ 4.346192] ubi0: scanning is finished
[ 4.403657] ubi0: attached mtd9 (name "rootfs", size 56 MiB)
[ 4.403689] ubi0: PEB size: 131072 bytes (128 KiB), LEB size: 126976 bytes
[ 4.408385] ubi0: min./max. I/O unit sizes: 2048/2048, sub-page size 2048
[ 4.415105] ubi0: VID header offset: 2048 (aligned 2048), data offset: 4096
[ 4.421945] ubi0: good PEBs: 448, bad PEBs: 0, corrupted PEBs: 0
[ 4.428697] ubi0: user volume: 4, internal volumes: 1, max. volumes count: 128
[ 4.434964] ubi0: max/mean erase counter: 3/0, WL threshold: 4096, image sequence number: 1011195579
[ 4.442010] ubi0: available PEBs: 0, total reserved PEBs: 448, PEBs reserved for bad PEB handling: 20
[ 4.451288] ubi0: background thread "ubi_bgt0d" started, PID 105
[ 4.455442] block ubiblock0_2: created from ubi0:2�[ 4.472218] VFS: Cannot open root device "ubiblock0_2" or unknown-block(31,2
[ 4.472250] Please append a correct "root=" boot option; here are the available partitions:
[ 4.479045] 1f00 256 mtdblock0 [ 4.487391] (driver?)
[ 4.492581] 1f01 128 mtdblock1 [ 4.493786] (driver?)
[ 4.498914] 1f02 384 mtdblock2 [ 4.500131] (driver?)
[ 4.505260] 1f03 64 mtdblock3 [ 4.506478] (driver?)
[ 4.511606] 1f04 64 mtdblock4 [ 4.512836] (driver?)
[ 4.517951] 1f05 64 mtdblock5 [ 4.519168] (driver?)
[ 4.524301] 1f06 960 mtdblock6 [ 4.525517] (driver?)
[ 4.530648] 1f07 64 mtdblock7 [ 4.531878] (driver?)
[ 4.536993] 1f08 64 mtdblock8 [ 4.538208] (driver?)
[ 4.543338] 1f09 57344 mtdblock9 [ 4.544555] (driver?)
[ 4.549685] fe00 2480 ubiblock0_2 [ 4.550902] (driver?)
[ 4.556204] Kernel panic - not syncing: VFS: Unable to mount root fs on unknown-block(31,9)
[ 4.557440] CPU2: stopping
[ 4.565674] CPU: 2 PID: 0 Comm: swapper/2 Not tainted 4.9.86 #0
[ 4.568452] Hardware name: Generic DT based system
[ 4.574300] [<c0215618>] (unwind_backtrace) from [<c0212010>] (show_stack+0x10/0x14)
[ 4.579146] [<c0212010>] (show_stack) from [<c0394dd4>] (dump_stack+0x7c/0x9c)
[ 4.587043] [<c0394dd4>] (dump_stack) from [<c021498c>] (handle_IPI+0xdc/0x184)
[ 4.594073] [<c021498c>] (handle_IPI) from [<c0209424>] (gic_handle_irq+0x80/0x8c)
[ 4.601279] [<c0209424>] (gic_handle_irq) from [<c021298c>] (__irq_svc+0x6c/0x90)
[ 4.608908] Exception stack(0xcf86df90 to 0xcf86dfd8)
[ 4.616460] df80: 00000001 00000000 00000000 c021a4c0
[ 4.621503] dfa0: ffffe000 00000000 c075efe4 00000004 00000000 c075f034 c0758a30 00000001
[ 4.629662] dfc0: 00000002 cf86dfe0 c020f5b4 c020f5a4 60000013 ffffffff
[ 4.637825] [<c021298c>] (__irq_svc) from [<c020f5a4>] (arch_cpu_idle+0x1c/0x38)
[ 4.644248] [<c020f5a4>] (arch_cpu_idle) from [<c024f5c8>] (cpu_startup_entry+0x108/0x1ac)
[ 4.651886] [<c024f5c8>] (cpu_startup_entry) from [<802094cc>] (0x802094cc)
[ 4.659949] CPU3: stopping
[ 4.666803] CPU: 3 PID: 0 Comm: swapper/3 Not tainted 4.9.86 #0
[ 4.669579] Hardware name: Generic DT based system
[ 4.675414] [<c0215618>] (unwind_backtrace) from [<c0212010>] (show_stack+0x10/0x14)
[ 4.680272] [<c0212010>] (show_stack) from [<c0394dd4>] (dump_stack+0x7c/0x9c)
[ 4.688168] [<c0394dd4>] (dump_stack) from [<c021498c>] (handle_IPI+0xdc/0x184)
[ 4.695198] [<c021498c>] (handle_IPI) from [<c0209424>] (gic_handle_irq+0x80/0x8c)
[ 4.702405] [<c0209424>] (gic_handle_irq) from [<c021298c>] (__irq_svc+0x6c/0x90)
[ 4.710034] Exception stack(0xcf86ff90 to 0xcf86ffd8)
[ 4.717588] ff80: 00000001 00000000 00000000 c021a4c0
[ 4.722634] ffa0: ffffe000 00000000 c075efe4 00000008 00000000 c075f034 c0758a30 00000001
[ 4.730789] ffc0: 00000003 cf86ffe0 c020f5b4 c020f5a4 60000013 ffffffff
[ 4.738954] [<c021298c>] (__irq_svc) from [<c020f5a4>] (arch_cpu_idle+0x1c/0x38)
[ 4.745374] [<c020f5a4>] (arch_cpu_idle) from [<c024f5c8>] (cpu_startup_entry+0x108/0x1ac)
[ 4.753012] [<c024f5c8>] (cpu_startup_entry) from [<802094cc>] (0x802094cc)
[ 4.761077] CPU0: stopping
[ 4.767930] CPU: 0 PID: 0 Comm: swapper/0 Not tainted 4.9.86 #0
[ 4.770706] Hardware name: Generic DT based system
[ 4.776540] [<c0215618>] (unwind_backtrace) from [<c0212010>] (show_stack+0x10/0x14)
[ 4.781403] [<c0212010>] (show_stack) from [<c0394dd4>] (dump_stack+0x7c/0x9c)
[ 4.789296] [<c0394dd4>] (dump_stack) from [<c021498c>] (handle_IPI+0xdc/0x184)
[ 4.796328] [<c021498c>] (handle_IPI) from [<c0209424>] (gic_handle_irq+0x80/0x8c)
[ 4.803536] [<c0209424>] (gic_handle_irq) from [<c021298c>] (__irq_svc+0x6c/0x90)
[ 4.811162] Exception stack(0xc075df60 to 0xc075dfa8)
[ 4.818722] df60: 00000001 00000000 00000000 c021a4c0 ffffe000 00000000 c075efe4 00000001
[ 4.823760] df80: 00000000 c075f034 c0758a30 00000001 00000000 c075dfb0 c020f5b4 c020f5a4
[ 4.831911] dfa0: 60000013 ffffffff
[ 4.840076] [<c021298c>] (__irq_svc) from [<c020f5a4>] (arch_cpu_idle+0x1c/0x38)
[ 4.843379] [<c020f5a4>] (arch_cpu_idle) from [<c024f5c8>] (cpu_startup_entry+0x108/0x1ac)
[ 4.851023] [<c024f5c8>] (cpu_startup_entry) from [<c071fc0c>] (start_kernel+0x368/0x3ec)
[ 4.859092] [<c071fc0c>] (start_kernel) from [<8020807c>] (0x8020807c)
[ 4.867332] Rebooting in 1 seconds..
Tim