microblaze: remove the explicit removal of system.dtb

I guess

    || (rm -f $@ && echo false)

... should be

    || (rm -f $@ && false)

In fact, no Makefile needs to delete a target explicitly on error.

It is automatically done since commit 9c2af1c7 ("kbuild: add
.DELETE_ON_ERROR special target").

I also reused equivalent cmd_shipped from scripts/Makefile.lib.
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>

arch/microblaze/boot/dts/Makefile

@@ -12,12 +12,9 @@ $(obj)/linked_dtb.o: $(obj)/system.dtb
 # Generate system.dtb from $(DTB).dtb
 ifneq ($(DTB),system)
 $(obj)/system.dtb: $(obj)/$(DTB).dtb
-	$(call if_changed,cp)
+	$(call if_changed,shipped)
 endif
 endif
 
-quiet_cmd_cp = CP      $< $@$2
-	cmd_cp = cat $< >$@$2 || (rm -f $@ && echo false)
-
 # Rule to build device tree blobs
 DTC_FLAGS := -p 1024