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>

arch/powerpc/boot/Makefile

@@ -315,8 +315,8 @@ $(addprefix $(obj)/, $(initrd-y)): $(obj)/ramdisk.image.gz
 $(obj)/zImage.initrd.%: vmlinux $(wrapperbits)
 	$(call if_changed,wrap,$*,,,$(obj)/ramdisk.image.gz)
 
-$(obj)/zImage.%: vmlinux $(wrapperbits)
-	$(call if_changed,wrap,$*)
+$(addprefix $(obj)/, $(sort $(filter zImage.%, $(image-y)))): vmlinux $(wrapperbits)
+	$(call if_changed,wrap,$(subst $(obj)/zImage.,,$@))
 
 # dtbImage% - a dtbImage is a zImage with an embedded device tree blob
 $(obj)/dtbImage.initrd.%: vmlinux $(wrapperbits) $(obj)/%.dtb