External Power off warnings %XPOFWARN

The Nordic-proprietary %XPOFWARN command controls the Power off warnings from the modem. The command is supported by nRF9160-SIxA-B1.v1.1.x≥3v1.2.x≥1

%XPOFWARN is based on voltage level detection without any delay. When this warning has been sent once, it needs to be enabled again by sending AT%XPOFWARN=1,<voltage>.