[POWERPC] ps3: add a default zImage target

Add a powerpc make target that can be loaded by the ps3 bootloader (kboot) and
set this as the default image to build for that platform.

Until the compressed zImage wrapper is made, this arranges for a stripped
vmlinux image to be built.

Signed-off-by: Geoff Levand <geoffrey.levand@am.sony.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>