I just setup a GFN boot onto rootfs RAID on Debian Stretch (which requires a seperate /boot partition for initramfs, kernel and device tree nonsense). uBoot reported that it couldn't find the .dtb file - /boot/dts/kirkwood-goflexhome.dtb, although it found and loaded kernel and initramfs.
I checked and double checked everything, tried recopying the dts files from my other machine. uBoot wasn't going to boot, nothing worked.
Also, on my Jessie laptop, fsck.ext4 reported that my fsck version is too old, mentioning "metadata_csum" (for the ext4 partition I was trying to boot from)
My solution - reformat the partition on a Jessie machine (ext4) and the problem went away.
From what I can gather, ext4 has been changed in Stretch ("Ext4 with new features"), and I think this is why uBoot wouldn't boot from my ext4 /boot partition.
I found a workaround here -
https://openvz.org/Installation_on_Debian_9#Ext4 ... although I haven't personally tested anything they are suggesting.
Also potentially worrying is the possibility that ext4 backward compatibility may be broken Stretch -> previous Debian releases, and presumably other linux distros ...
Don Charisma ... because anything is possible with Charisma
My blog -
http://DonCharisma.org
Our commercial site -
http://DonCharisma.com