[patch] fix "partition-zero"/%BOOT booting for PowerPC targets.

[Andrei E. Warkentin]

Hi,

This is a patch against r1272 which implements fully bootcode booting via
Apple boot partition descriptors. The current implementation in
libopenbios/bootcode_load.c hardcoded values for QUIK (the old
OldWorld bootloader), which is useless if you try to boot anything
else.

In particular, I implemented this to simplify my development and test
cycle on iQUIK (https://github.com/andreiw/quik), but I also tested
this with the NetBSD floppy. The later now boots to stage 2 as well,
although it tries using some functionality unavailable in the OpenBIOS
implementation when compared to Apple's.

I did this in the simplest way possible.

Btw, I see that the optimization bugs are still present. I had to
compile with -O0, otherwise the resulting image doesn't boot (GCC 4.8
here).