snippets: support for EXTRA_DTC_OVERLAY_FILE and EXTRA_CONF_FILE

This commit adds change snippets to use the new EXTRA_DTC_OVERLAY_FILE
and EXTRA_CONF_FILE Zephyr build system variables instead of
DTC_OVERLAY_FILE and OVERLAY_CONFIG.

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