kbuild: move cmd_mkimage to scripts/Makefile.lib

Because cmd_mkimage is used in various subdirectories,
it seems reasonable to define it in scripts/Makefile.lib.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
diff --git a/board/cray/L1/Makefile b/board/cray/L1/Makefile
index 5540298..716a5a3 100644
--- a/board/cray/L1/Makefile
+++ b/board/cray/L1/Makefile
@@ -15,13 +15,9 @@
 $(obj)/bootscript.c: $(obj)/bootscript.image $(src)/x2c.awk
 	$(call cmd,awk)
 
-quiet_cmd_mkimage = MKIMAGE $@
-cmd_mkimage = $(objtree)/tools/mkimage $(MKIMAGEFLAGS_$(@F)) -d $< $@ \
-        $(if $(KBUILD_VERBOSE:1=), >/dev/null)
-
 MKIMAGEFLAGS_bootscript.image := -A ppc -O linux -T script -C none \
 						-a 0 -e 0 -n bootscript
 $(obj)/bootscript.image: $(src)/bootscript.hush
 	$(call cmd,mkimage)
 
-clean-files := bootscript.c bootscript.image
\ No newline at end of file
+clean-files := bootscript.c bootscript.image