Makefile: Allow inclusion of source files from 3rdparty/

Change-Id: I81c6f628f239223ba293a1196f70e4f26e022f6c
Signed-off-by: Nico Huber <nico.huber@secunet.com>
Reviewed-on: https://review.coreboot.org/16950
Tested-by: build bot (Jenkins)
Reviewed-by: Martin Roth <martinroth@google.com>
1 file changed