kbuild: fix make rpm for powerpc

The default target for most powerpc platforms is zImage.  The
zImage however is in arch/powerpc/boot and the mkspec script
was set up to get the kernel from the top level of the kernel
tree.  This patch copies vmlinux to arch/powerpc/boot and then
copies the kernel to the tmp directory so the rpm can be made.

Signed-off-by: Mike Wolf <mjw@us.ibm.com>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>