Port yaboot logic for various powerpc machine types

Port yaboot logic for various powerpc machine types

Some powerpc machines require not updating the NVRAM, and some require
adding a CHRP note.  This can be handled by existing grub-install
command-line options, but it's friendlier to detect this automatically.

Forwarded: no
Last-Update: 2014-01-28

Patch-Name: install_powerpc_machtypes.patch