Updates:
My conclusion around the USB part was completely wrong. The device is available it just needs a bit of time to initialize.
The main delay finding it was due to a nasty bug in the block device layer of the linux kernel (maybe only in tilegx ARCH).
If you try to boot initramfs (cpio xz'ed image included in the kernel ELF) you might see this error:
[ 6.705965] Unpacking initramfs...
[ 6.710258] Initramfs unpacking failed: junk in compressed archivehidden in the history due to panic on unknown block device:
[ 6.781978] List of all partitions:
[ 6.784802] No filesystem could mount root, tried:
[ 6.790076] Kernel panic - not syncing: VFS: Unable to mount root fs on unknown-block(0,0)
[ 6.799139] ---[ end Kernel panic - not syncing: VFS: Unable to mount root fs on unknown-block(0,0)
Disable "Enable the block layer" in the main kernel menu (CONFIG_BLOCK) and initramfs will work fine again.
After this fix I added a very limited busybox image (cross-compiled for tilegx), giving me at least some automatic output like lsusb, ps, ...
Serial input is not working, this may be due to some missing mikrotik kernel stuff (GPIO/UART/... sharing?) but at least we can see the output of custom sh scripts:
http://pastebin.com/W4JX824h
The nice surprise was that USB kicked in without notice, after some testing I concluded that we need at least 10sec wait before we try to use it as root device, linux append option rootdelay=15 works like expected:
[ 6.320838] Kernel command line: init=/sbin/init rootdelay=15
[ 6.823831] ohci-pci: OHCI PCI platform driver
[ 6.829225] Waiting 15 sec before mounting root device...
[ 11.295838] usb 1-1: new high-speed USB device number 2 using tilegx-ehci
[ 11.411313] usb 1-1: New USB device found, idVendor=1221, idProduct=3234
[ 11.411359] usb 1-1: New USB device strings: Mfr=1, Product=2, SerialNumber=3
[ 11.411400] usb 1-1: Product: USB Disk
[ 11.411425] usb 1-1: Manufacturer: Flash
[ 11.411452] usb 1-1: SerialNumber: ****
[ 21.829220] devtmpfs: mounted
Next step is to remove my initramfs (anyhow broken if I want to enable partitions/block devices in the kernel) and move the busybox install to this USB drive.
This should give me enough space for more tooling, just need to find a fix for the serial input issue.