[POWERPC] Skip the "copy down" of the kernel if it is already at zero.

This patch allows the kernel to recognized that it was loaded at zero
and the copy down of the image is unnecessary.  This is useful for
Simulators and kexec models.
On a typical 3.8 MiB vmlinux.strip this saves about 2.3 million instructions.

Signed-off-by: Paul Mackerras <paulus@samba.org>