nrf: implement supervisor.Runtime.serial_connected

[dhalbert]

RIght now nrf/common-hal/supervisor/Runtime.c serial_connected() always returns true.

[arturo182]

Not when using USB: https://github.com/adafruit/circuitpython/blob/master/ports/nrf/supervisor/serial.c#L94 And for HW UART I'm not sure it's possible to detect it.

[dhalbert]

OK great, I only saw the first definition in that file.