Support UART Break Condition

[philwareublox]

Many MCU UARTs can detect a 'break condition' where the Rx line is held low for longer than a frame period. https://en.wikipedia.org/wiki/Universal_asynchronous_receiver-transmitter#Break_condition

Our cellular modules will perform this break condition when going into PSM and shutting down.

I believe ESP32, nRF52 and STM32 MCUs support a break interrupt. We should investigate how we could use this break condition to detect PSM and power down states, for when V_INT monitoring is not possible, or +UPSMR: 1 URC is not implemented.

[RobMeades]

Excellent idea: will raise an internal JIRA to track this also.