Makefile: Add support for out-of-tree sources

Change-Id: If029c713a06a77aa8001cb4093806580086e3cbe
Signed-off-by: Nico Huber <nico.huber@secunet.com>
Reviewed-on: https://review.coreboot.org/18358
Tested-by: Nico Huber <nico.h@gmx.de>
Reviewed-by: Adrian-Ken Rueegsegger <ken@codelabs.ch>
1 file changed