Makefile: pass external DTB provided by make variable EXT_DTB to binman

Yocto build is using EXT_DTB to repack U-Boot during signed
FIT kernel creation. First, it takes the DTB from U-Boot, then
it creates signed FIT kernel image and places kernel key into
the DTB. After that it executes:
    make EXT_DTB=/path/to/dtb/with/kernel/key

Despite the fact that in the documentation all examples with EXT_DTB
use only single DTB, the patch allows pass many DTB files separated
by spaces, e.g.:
    make EXT_DTB="/path1/to/1.dtb /path2/to/2.dtb"

The first DTB from the list become default configuration.

Signed-off-by: Alexander Kochetkov <al.kochet@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>