[PATCH] powerpc/boot: Better use of defaultimage-

The following reworks how defaultimage- is used.  We default to zImage
here and then override it on platforms that need something more (uImage
in the future) or less (vmlinux on iSeries).

Signed-off-by: Tom Rini <trini@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>