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/scripts/Makefile.lib b/scripts/Makefile.lib
index a04439dc..c24c5e8 100644
--- a/scripts/Makefile.lib
+++ b/scripts/Makefile.lib
@@ -379,3 +379,11 @@
 cmd_xzmisc = (cat $(filter-out FORCE,$^) | \
 	xz --check=crc32 --lzma2=dict=1MiB) > $@ || \
 	(rm -f $@ ; false)
+
+# Additional commands for U-Boot
+#
+# mkimage
+# ---------------------------------------------------------------------------
+quiet_cmd_mkimage = MKIMAGE $@
+cmd_mkimage = $(objtree)/tools/mkimage $(MKIMAGEFLAGS_$(@F)) -d $< $@ \
+	$(if $(KBUILD_VERBOSE:1=), >/dev/null)