[POWERPC] ps3: add a default zImage target

It's currently not possible to build the default zImage
target if PS3 is the only selected platform. This is
a hack to fall back to building the pseries style
zImage, so the build is successful. This will probably
change in the future, if someone writes a PS3 specific
boot wrapper.

Signed-off-by: Arnd Bergmann <arnd.bergmann@de.ibm.com>