Provide a means for the application to configure battery voltage for DeviceStatusAns

[janakj]

This module contains the code to measure batter and temperature levels. The measured battery voltage currently doesn't appear to be integrated into LoRaWAN. Check if this code works correctly and maybe integrate it into the device reporting MAC command.

[janakj]

Instead of integrating the code that measures battery voltage with an ADC, it might be more flexible to implement a new AT command AT$BAT=value and let the application set the battery level via this AT command. This approach will allow the application to apply custom battery voltage measuring techniques and to perform platform-specific conversions. Suggested by @hubmartin.

In addition to that, it might also make sense to implement a new event notification command that would be emitted by the modem once the transmitter has been activated. The application could then use the notification to properly time battery voltage measurements during LoRa transmissions. It should be possible to turn the event notification on or off from the application.