Extend usage for OF_OVERLAY_LIST beyond SPL

Extend usage for OF_OVERLAY_LIST beyond SPL

Allow to use OF_OVERLAY_LIST also for the case that the overlays just
need be built, e.g. when they will be picked up by binman as artifacts
of the final U-Boot image. The IOT2050 boards have such a need when
switching to OF_UPSTREAM.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Reviewed-by: Sumit Garg <sumit.garg@linaro.org>
Reviewed-by: Simon Glass <sjg@chromium.org>