Add out-of-source U-Boot device tree support

Similaly to Linux, this patch adds the ability to copy in and build
out-of-source device tree sources during a U-Boot build.

Signed-off-by: James Balean <james@balean.com.au>
Cc: Arnout Vandecappelle <arnout@mind.be>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>