powerpc/boot: Don't install zImage.* from make install

powerpc/boot: Don't install zImage.* from make install

in commit 29f1aff2c (powerpc: Copy bootable images in the default
install script) we changed to copying all the built boot targets based
on the assumption that it's backwards compatible.  It turns out that
debian devived installkernel scripts will barf if not given exactly 4
args.

This change reverts make install to just install the vmlinux (we can
change the dfault in a seperate patch) and introduces a new make
zInstall which works with a more flexible installkernel script.

Cc: Grant Likely <grant.likely@secretlab.ca>
Signed-off-by: Tony Breeds <tony@bakeyournoodle.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>