[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/Makefile b/arch/powerpc/Makefile
index 26788de..7949920 100644
--- a/arch/powerpc/Makefile
+++ b/arch/powerpc/Makefile
@@ -166,6 +166,9 @@
   @echo '  *_defconfig     - Select default config from arch/$(ARCH)/configs'
 endef
 
+install:
+	$(Q)$(MAKE) $(build)=$(boot) BOOTIMAGE=$(KBUILD_IMAGE) install
+
 archclean:
 	$(Q)$(MAKE) $(clean)=$(boot)