Support external boards

it works!
```sh
soldair@mintgor~/Projects/opensource/leo (support-external-boards)☘ sudo npm install -g leo-board-pinoccio
npm http GET https://registry.npmjs.org/leo-board-pinoccio
npm http 200 https://registry.npmjs.org/leo-board-pinoccio
npm http GET https://registry.npmjs.org/leo-board/0.0.1
npm http 200 https://registry.npmjs.org/leo-board/0.0.1
npm http GET https://registry.npmjs.org/leo-board/-/leo-board-0.0.1.tgz
npm http 200 https://registry.npmjs.org/leo-board/-/leo-board-0.0.1.tgz
leo-board-pinoccio@0.0.0 /usr/local/lib/node_modules/leo-board-pinoccio
└── leo-board@0.0.1
soldair@mintgor~/Projects/opensource/leo (support-external-boards)☘ node
> var boards = require('./boards')
undefined
> boards.pinoccio
{ upload: 
   { tool: 'avrdude',
     protocol: 'arduino',
     maximum_size: 253952,
     speed: 115200 },
  pid: {},
  vid: {},
  bootloader: 
   { tool: 'avrdude',
     low_fuses: '0xDE',
     high_fuses: '0xD0',
     extended_fuses: '0xFE',
     unlock_bits: '0x3F',
     lock_bits: '0x2F',
     file: 'STK500RFR2/release_0.51/boot_pinoccio.hex' },
  build: 
   { mcu: 'atmega256rfr2',
     f_cpu: '16000000L',
     board: 'PINOCCIO',
     core: 'arduino',
     variant: 'pinoccio',
     extra_flags: '-D__PROG_TYPES_COMPAT__' },
  name: 'Pinoccio Scout',
  cpu: 'atmega256rfr2' }
> 
```