Update board name to identify feather m0

Currently the board is set to `SAMD_ZERO`. I think it should be set to something that indicates the board is a feather m0 like `SAMD_FEATHER_M0`.

https://github.com/adafruit/ArduinoCore-samd/blob/d8f4306d0f5c94951bb761b267480623fd0b9239/boards.txt#L33

This would help when trying to make libraries that need to account for differences between the Zero and Feather M0.
