powerpc: Make COFF zImages for old 32-bit powermacs

This adds code to build zImage.coff and/or zImage.initrd.coff when
CONFIG_PPC32 and CONFIG_PPC_PMAC are defined.  It also restructures
the OF client code and adds some workarounds for OF quirks on the
older machines.

Signed-off-by: Paul Mackerras <paulus@samba.org>