Facilitate external use of the project

Make the Zephyr discovery more robust.

Allow setting CycloneDDS as a Zephyr module with ZEPHYR_EXTRA_MODULES
outside of the project.

Allow setting OVERLAY_CONFIG for the Zephyr app outside of the project.

Issue-Id: SCM-5754
Signed-off-by: Ambroise Vincent <ambroise.vincent@arm.com>
Change-Id: I5954eda85fba798b2dee8edd7517de7891178ecf