Add AT command to lock/unlock parameters configurable over the network

[janakj]

Many MAC parameters on the node can be remotely changes from the LoRaWAN network server: channel mask, channel parameters, data rate, TX power, RX & RX2 parameters, retry count, aggregated duty cycle, ADR ACK parameters. This could lead to problems if the LNS is not configured properly, or if ADR cannot cope with a particular deployment scenario.

To make the node more robust, we should consider adding an API to lock or unlock individual parameters:

AT$LOCK CHMASK 1    # Channel mask can be updated by the network
AT$LOCK DR 0        # Data rate cannot be updated by the network