cmake: introduce EXTRA_DTC_OVERLAY_FILE for devicetree

This commit introduces EXTRA_DTC_OVERLAY_FILE.

This allows users to specify additional devicetree overlays in addition
to a sample / board specific default devicetree overlay files.

It also allows snippets to provide extra devicetree overlays on addition
to sample / board specific overlays.

The name EXTRA_DTC_OVERLAY_FILE clearly indicates the purpose and
follows the naming scheme of EXTRA_ in front of variable name.

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