powerpc: Avoid circular dependency with zImage.%

The rule to create the final images uses a zImage.% pattern.
Unfortunately, this also matches the names of the zImage.*.lds linker
scripts, which appear as a dependency of the final images. This somehow
worked when $(srctree) used to be an absolute path, but now the pattern
matches too much. List only the images from $(image-y) as the target of
the rule, to avoid the circular dependency.

Reported-and-tested-by: Mike Qiu <qiudayu@linux.vnet.ibm.com>
Signed-off-by: Michal Marek <mmarek@suse.cz>