Use search path when building dependencies

clang is more picky on that.

Change-Id: Iaa8472beb6e275c39037d11e1a72dbb80d46424b
Signed-off-by: Patrick Georgi <patrick@georgi-clan.de>
Reviewed-on: http://review.coreboot.org/779
Tested-by: build bot (Jenkins)
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>