dtc: mkimage: Add the possibility to specify DTC

FreeBSD recently switch to it's BSDL dtc. While it support most of the
features of the GPL one it still lacks the incbin directive.
Add the possibility to specify which dtc we want to use for compiling dts
and generating fit image.

Signed-off-by: Emmanuel Vadot <manu@bidouilliste.com>
Acked-by: Simon Glass <sjg@chromium.org>