Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Forced commit to record that this file was repocopied from
src/sys/boot/powerpc/loader.

Repocopy by: simon@

"Ok, my loader's now up to putting up a prompt. It probes disks partially
but can't boot from them yet."

Thanks to Stephane Potvin for the some of the code in this set.

Submitted by: Benno Rice <benno@jeamland.net>