lpc: power-up som on start-up when batteries are disconnected

For automated testing, i want the reform to start-up when it gets powered from the barrel jack, but only when all the batteries are disconnected.

Doing this only with the batteries un-plugged is important as automatic power-up with batteries connected can deep discharge them when external power is lost and the lpc goes into brown-out condition; continuously trying to start the som after a brown-out until all the batteries are discharged.