kbuild: use cmd_shipped instead of cmd_copy

We already have cmd_shipped in scripts/Makefile.lib.
Use it rather than defining a new command cmd_copy.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
diff --git a/dts/Makefile b/dts/Makefile
index e59550c..3fca5f5 100644
--- a/dts/Makefile
+++ b/dts/Makefile
@@ -14,11 +14,8 @@
 
 DTB := arch/$(ARCH)/dts/$(DEVICE_TREE).dtb
 
-quiet_cmd_copy = COPY    $@
-      cmd_copy = cp $< $@
-
 $(obj)/dt.dtb: $(DTB) FORCE
-	$(call if_changed,copy)
+	$(call if_changed,shipped)
 
 targets += dt.dtb