NetBSD/powerpc boot program. Highligts:
- Rearrange a bit so that we can sanely add boot programs for
non-OpenFirmware systems.
- Add support for loading compressed kernels (for booting from
1.44M floppies).
- Add support for loading Elf kernels (Elf is the new standard format
for NetBSD/powerpc). (XXX Don't load symbols right now - the code
to do it is there, but I don't particularly like how space for the
symbol table is allocated; I will revisit this soon'ish).
