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>