devicetree: remove support for DTC_OVERLAY_FILE in environment

Setting of DTC_OVERLAY_FILE as an environment setting was deprecated
before Zephyr 1.14 LTS.

This commit remove the support for this possibility and thus cleans up
the build system.

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