kboot: Allow loading fdt from different sources

Linux has /sys/firmware/fdt and /proc/device-tree to publish the dtb for
the system. The former has it all in one file, while the latter breaks
it out. Prefer the former since it's the more modern interface, but
retain both since I don't have a PS3 to test to see if its kernel is new
enough for /sys/firmware or not.

In addition, do the proper fixup.

Sponsored by: Netflix

(cherry picked from commit 111610316e49dd6dc6b0b74455c1e4c81960faa8)