Thanks for the link - I nailed it down to:
linux-kirkwood-3.13.7-1-arm.pkg.tar.xz - OK
linux-kirkwood-3.13.9-2-arm.pkg.tar.xz - FAIL
The only differences I find are when starting up
3.13.7:
$this->bbcode_second_pass_code('', '
[ 15.208663] Security Framework initialized
[ 15.208689] AppArmor: AppArmor disabled by boot time parameter
[ 15.208752] Mount-cache hash table entries: 512
[ 15.209340] Initializing cgroup subsys devices
')
versus 3.13.9:
$this->bbcode_second_pass_code('', '
[ 14.642405] Security Framework initialized
[ 14.642431] AppArmor: AppArmor disabled by boot time parameter
[ 14.642497] Mount-cache hash table entries: 1024 (order: 0, 4096 bytes)
[ 14.642514] Mountpoint-cache hash table entries: 1024 (order: 0, 4096 bytes)
[ 14.643094] Initializing cgroup subsys devices
')
And then of course the device-detection:
3.13.7
$this->bbcode_second_pass_code('', '
[ 18.314812] Key type dns_resolver registered
[ 18.319663] registered taskstats version 1
[ 18.341041] mmc0: new high speed SDIO card at address 0001
[ 18.514039] usb 1-1: new high-speed USB device number 2 using orion-ehci
[ 18.685289] hub 1-1:1.0: USB hub found
[ 18.694028] hub 1-1:1.0: 4 ports detected
[ 18.984266] usb 1-1.1: new high-speed USB device number 3 using orion-ehci
[ 19.116817] usb-storage 1-1.1:1.0: USB Mass Storage device detected
[ 19.134121] scsi1 : usb-storage 1-1.1:1.0
[...]
[ 20.664046] sd 2:0:0:0: [sdc] Attached SCSI disk
[ 21.864857] UKSM: relative memcmp_cost = 2600 hash=234621611 cmp_ret=0.
[ 21.878677] console [netcon0] enabled
[ 21.882355] netconsole: network logging started
[ 21.887033] rtc-mv rtc-mv: setting system clock to 2014-06-19 08:21:05 UTC (1403166065)
[ 21.936682] EXT3-fs (sdc5): error: couldn't mount because of unsupported optional features (240)
[ 21.946412] EXT2-fs (sdc5): error: couldn't mount because of unsupported optional features (240)
[ 22.010616] EXT4-fs (sdc5): mounted filesystem with ordered data mode. Opts: (null)
[ 22.018361] VFS: Mounted root (ext4 filesystem) on device 8:37.
[ 22.059576] devtmpfs: mounted
')
3.13.9
$this->bbcode_second_pass_code('', '
[ 17.758570] Key type dns_resolver registered
[ 17.763422] registered taskstats version 1
[ 17.768605] console [netcon0] enabled
[ 17.772322] netconsole: network logging started
[ 17.776965] rtc-mv rtc-mv: setting system clock to 2014-06-19 08:27:10 UTC (1403166430)
[ 17.785533] VFS: Cannot open root device "sdc5" or unknown-block(0,0): error -6
[ 17.792969] Please append a correct "root=" boot option; here are the available partitions:
[ 17.801424] Kernel panic - not syncing: VFS: Unable to mount root fs on unknown-block(0,0)
[ 17.809726] CPU: 0 PID: 1 Comm: swapper Not tainted 3.13.9-2-ARCH #1
')
Now I'm off trying to build a 3.13.7-kernel, hoping it'll work, then try the 3.13.9 and see what's wrong... Could take a couple of days, so if somebody else has any idea in the meantime.