Add Board Manager Updates

[toddtreece]

Changes

• Adds Bluefruit Micro to boards.txt
• Adds bluefruitmicro variant
• Adds build_package.sh bash script to package the core for the Adafruit package index

  Running a Package Build

Run the build_package.sh script and enter a new version:

$ ./build_package.sh 
VERSION [1.3.0]: 1.3.0

The resulting archive will then be available in the build/ folder along with the JSON needed to add to the package index file in the adafruit/arduino-board-index repo:

├── build
│   ├── adafruit-1.3.0.tar.bz2
│   └── package.json