Adding Adafruit CLUE Alpha

[bgould]

https://www.adafruit.com/clue

They note that this is an alpha release of the board and that the pinout/sensors could change, so I've named this target "clue_alpha" in case such changes do occur.

[aykevl]

Thanks!
So of course the actual hardware produced never goes out of "alpha" - it's what it is today. So we can't just drop support for that. Therefore, I would like to propose the following for when this board comes out of the alpha phase:

• If everything stays the same (or compatible), rename it to clue and provide a clue_alpha.json file for compatibility that simply inherits the clue.json file: {"inherits": ["clue"]}.
• If the board changes after the alpha period, keep both versions around.