sysbuild: Allow to use file suffix DTS overlays

Allow to add an image-specific DTS overlay, that is applied based on the
FILE_SUFFIX.
Up to this point, it was possible to specify such Kconfig files
(sysbuild/<image>_<file_suffix>.conf),
but it did not work with dts overlays
(sysbuild/<image>_<file_suffix>.overlay).
This commit aligns the logic of finding configuration files for both
scenarios mentioned above.

Ref: NCSDK-NONE

Signed-off-by: Tomasz Chyrowicz <tomasz.chyrowicz@nordicsemi.no>