build: allow user to specify DTC_OVERLAY_FILE directly

In some cases, it's possible to share a DTC overlay across multiple
boards (one example is when the overlay just has to choose a
zephyr,code-partition and set it to a well-known value).

To support this, only go looking around in DTC_OVERLAY_DIR if
DTC_OVERLAY_FILE is not already given.

Signed-off-by: Marti Bolivar <marti.bolivar@linaro.org>