powerpc: Make building the boot image work for both 32-bit and 64-bit

At the moment we don't have a merged arch/powerpc/boot, so we build the
boot images in arch/ppc/boot and arch/ppc64/boot.  Unfortunately the
makefile targets are different in those two directories, so this makes
a change to accommodate both for the moment.

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