cmake: Look for revision specific overlays also in <app>/boards/

This is a follow-up to commit 77ecd683.

Add missing check that looks for a revision specific overlay located
in the `<app>/boards/` directory similar to how overlays and Kconfig
fragments are handled.

Signed-off-by: Andrzej Głąbek <andrzej.glabek@nordicsemi.no>
Signed-off-by: Torsten Rasmussen <Torsten.Rasmussen@nordicsemi.no>