[POWERPC] Enable make install

make help on powerpc says make install is available.
But it failed due to no rule to make install.

This patch enables make install to work.

Signed-off-by: Akinobu Mita <mita@fixstars.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>
diff --git a/arch/powerpc/boot/Makefile b/arch/powerpc/boot/Makefile
index 3e172ff..1269b49 100644
--- a/arch/powerpc/boot/Makefile
+++ b/arch/powerpc/boot/Makefile
@@ -176,7 +176,7 @@
 $(obj)/zImage.initrd:	$(addprefix $(obj)/, $(initrd-y))
 	@rm -f $@; ln $< $@
 
-install: $(CONFIGURE) $(image-y)
+install: $(CONFIGURE) $(addprefix $(obj)/, $(image-y))
 	sh -x $(srctree)/$(src)/install.sh "$(KERNELRELEASE)" vmlinux System.map "$(INSTALL_PATH)" $<
 
 # anything not in $(targets)