cmake: extend zephyr_file(CONF_FILES ...) to take NAMES as argument

Extend zephyr_file(CONF_FILES ...) to take a NAMES list of file names
to find instead of creating file names based on board and revision.

This allows to unify lookup prj.conf and <board>/app.overlay for
application, as well as pave the way for future enhancements to
configuration file handling.

Signed-off-by: Torsten Rasmussen <Torsten.Rasmussen@nordicsemi.no>