Add board identifier for Uno WiFi Rev2

[brentru]

AdafruitIO_Board type is currently unknown for the Arduino Uno WiFi Rev2. Add support for this board identifier to _board.cpp.

Product: https://store.arduino.cc/usa/arduino-uno-wifi-rev2

Relevant code segment(s):
https://github.com/adafruit/Adafruit_IO_Arduino/blob/master/src/util/AdafruitIO_Board.cpp#L57

Note: Support for float formatting was added in PR #97