Makefile: Prioritize external dtb if defined

Prioritize external dtb if its passed via EXT_DTB
than the dtb that was built in the tree. With this
patch it appends the specified external dtb to
the u-boot image.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Signed-off-by: Siva Durga Prasad Paladugu <siva.durga.paladugu@xilinx.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/Makefile b/Makefile
index 059978b..6eb08a5 100644
--- a/Makefile
+++ b/Makefile
@@ -1071,7 +1071,7 @@
 u-boot-fit-dtb.bin: u-boot-nodtb.bin $(EXT_DTB)
 		$(call if_changed,cat)
 else
-u-boot-fit-dtb.bin: u-boot-nodtb.bin $(FINAL_DTB_CONTAINER)
+u-boot-fit-dtb.bin: u-boot-nodtb.bin fit-dtb.blob $(FINAL_DTB_CONTAINER)
 	$(call if_changed,cat)
 endif