cmake: extend zephyr_file(CONF_FILES <paths>) to allow a list of paths

Extend zephyr_file(CONF_FILES <paths>) to take a list of paths to lookup
instead of a single path.

This remove the need of callers to do:
> foreach(p ${paths})
>   zephyr_file(CONF_FILES ${p})
>   ...
> endforeach()

and instead allow them to just pass the list directly to
> zephyr_file(CONF_FILES ${paths})

In addition the help text is updated with the detail that CONF_FILES can
be given an empty list. This has always been possible, but not
described.

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