I haven't actually flashed the boot image - I use "fastboot boot mmcblk1p1.img" instead. And I'm using the stock .dtb.
Oh, I also formatted the sdcard on my PC, probably with a live system. I have noticed that the Shield and PCs seem to format cards differently - PCs usually leave a 1 MB gap before the first partition to prevent any alignment issues, while the Shield seems to format without a gap. I think the gap is required for Linux, even tho' Android will refuse to mount it.
Edit: added more info and a screenshot
I'm actually trying to set this up in recovery partition, so can cycle through OS'es with adb commands, I also tried to fastboot boot either.
Maybe I'm asking too much, but is it possible for you to dump image(dd then zip) from your working rootfs ?
Thanks for all your troubles.