payloads/external/Makefile: Add missing dependency

The depthcharge .config only exists after depthcharge was built. Other
possible `PAYLOAD_CONFIG` files most probably miss a rule, too.

Change-Id: I8e6f79bb2bd07cbef1317f2623bbef9ca0e74880
Signed-off-by: Nico Huber <nico.huber@secunet.com>
Tested-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-on: https://review.coreboot.org/22137
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>
1 file changed