Adopt for Adafruit Feather 32u4 with integrated LoRa chip (RFM95)

[joergkeller]

• Check if LMIC library works for that board (https://learn.adafruit.com/the-things-network-for-feather)
• Remove feather32u4 compile issue USBDevice.init
• Reduce code size for feather32u4

[joergkeller]

RTC alarm could replace the missing timer: https://github.com/arduino-libraries/RTCZero