[POWERPC] Create a zImage for legacy iSeries

This zImage is really just the stripped vmlinux, but it means that there
is one less special case for iSeries and also that the zImages will be
built for a combined kernel build that happens to include iSeries.

This zImage boots fine on legacy iSeries.

Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Paul Mackerras <paulus@samba.org>