[POWERPC] Fix build of cell zImage.initrd

The patch adding support for zImage.ps3 didn't add a zImage.initrd.ps3
target causing builds of zImage.initrd to fail when ps3 is included in
the .config. The current method of generating ps3 images doesn't support
initrd's yet, so we create a dummy target that only displays a warning
message instead.
Signed-off-by: NBenjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: NPaul Mackerras <paulus@samba.org>

arch/powerpc/boot/Makefile

@@ -152,6 +152,9 @@ $(obj)/zImage.initrd.miboot: vmlinux $(wrapperbits)
 $(obj)/zImage.ps3: vmlinux
 	$(STRIP) -s -R .comment $< -o $@
 
+$(obj)/zImage.initrd.ps3: vmlinux
+	@echo "  WARNING zImage.initrd.ps3 not supported (yet)"
+
 $(obj)/uImage: vmlinux $(wrapperbits)
 	$(call cmd,wrap,uboot)