need to add RTC library

[bombasticbob]

apparently the Zero (and others) have an RTC library

https://www.arduino.cc/en/Tutorial/SleepRTCAlarm

need to incorporate this API into the xmega code

[bombasticbob]

the library is documented here:

https://www.arduino.cc/en/Reference/RTC

[bombasticbob]

the source is here:

https://github.com/arduino-libraries/RTCZero/tree/master/src