Add MCP2221

[caternuson]

Add support for Microchip MCP2221 special case controlled by OS environment variable.

NOTE - also reworked the general logic in the "special cases" section to remove the try/except blocks.

[ladyada]

needs a lint - also please add a check in the tester sketch that will print out if platformdetect found an mcp2221

[ladyada]

https://github.com/adafruit/Adafruit_Python_PlatformDetect/blob/master/bin/detect.py

[caternuson]

Should I have put one of those in for FT232H also?

[ladyada]

yah why not :)

[caternuson]

I wonder if we should have board.id be something like SPECIAL_CASE for any of these OS env var set cases? And then chip.id would be the actual chip, like FTDI_FT232H and MICROCHIP_MCP2221.

[ladyada]

can do that in a future PR - good enough for now!

[caternuson]

cool. works for me.

i'll bump and release.