Implemented pluggable discovery board matching

[cmaglie]

The implementation follows the pluggable discovery specification.

Now the board identification is done on myboard.upload_port.N.PROP definitions in boards.txt so, the old:

myboard.vid.0=0x1234
myboard.pid.0=0x4567

should be changed to:

myboard.upload_port.0.vid=0x1234
myboard.upload_port.0.pid=0x4567

Anyway, to keep backward compatibility all the existing non-pluggable platforms are automatically and transparently converted to the new format.

see arduino/tooling-rfcs#2

[silvanocerza]

Just some small changes.