cmake: introduce EXTRA_CONF_FILE and deprecate OVERLAY_CONFIG

This commit introduces EXTRA_CONF_FILE and deprecates OVERLAY_CONFIG.

It has often caused confusion that OVERLAY_CONFIG adds extra
configuration fragments to the value of CONF_FILE (default: prj.conf),
but the similar named variable DTC_OVERLAY_FILE replaces the default
dtc overlay file used by the build system.

To remove such confusion, this commit introduces the EXTRA_ prefix in
front of CONF_FILE to clearly indicate it's purpose.

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