build: Overlay build vars expand ${ZEPHYR_<module>_MODULE_DIR}

Support referencing module directories by name in CONF_FILE,
OVERLAY_CONFIG, and DTC_OVERLAY_FILE so that projects can reference
overlay files in arbitrary modules.

Verified by passing all the following tests:
  ./scripts/twister -T tests/cmake/overlays/

Fixes #41830

Signed-off-by: Gregory Shue <gregory.shue@legrand.us>
15 files changed