flashing feather m0 on OS X El Capitan

[g-ollivier]

This micropython version for M0 feathers is great news !

The tools/bossac_osx does not work on my Mac (OSX El Capitan). So I managed to rebuild it from https://github.com/shumatech/BOSSA.

Then, in order to avoid a "not found" error message, it seems to be easier with this track

• connect a lipo to the feather
• connect the feather to the mac
• reset the board by connecting a 10 kOhm resistor between Reset and Gnd pins
  and then launch the bossac tool.

This way, I managed to flash a feather m0 basic proto and a feather m0 adalogger.