powerpc: Add missing cuImage.initrd.% target

This target is needed to build cuImages with an embedded ramdisk image.
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

arch/powerpc/boot/Makefile

@@ -321,6 +321,9 @@ $(obj)/zImage.iseries: vmlinux
 $(obj)/uImage: vmlinux $(wrapperbits)
 	$(call if_changed,wrap,uboot)
 
+$(obj)/cuImage.initrd.%: vmlinux $(obj)/%.dtb $(wrapperbits)
+	$(call if_changed,wrap,cuboot-$*,,$(obj)/$*.dtb,$(obj)/ramdisk.image.gz)
+
 $(obj)/cuImage.%: vmlinux $(obj)/%.dtb $(wrapperbits)
 	$(call if_changed,wrap,cuboot-$*,,$(obj)/$*.dtb)