cmake: extend zephyr_file(<mode>) to support searching of config files

This commit extends zephyr_file(<mode>) to support `CONF_FILES` which
allows for finding and appending Zephyr Kconfig fragments and DTS
overlays.

Introducing mode:CONF_FILES allows have a uniform way of looking up
<board>.conf and <board>.overlay, and ensure that extending such
functionality keeps identical behaviour in shields, tests, and samples.

Using zephyr_file(CONF_FILES) a lot of duplicated code can be removed.

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