Boards calibration

[marcomicera]

core/server/config.json has a new optional section:

"calibration": {
    "calibration_device": "12:34:56:78:9a:bc",
    "min_num_calibration_messages": 2,
    "placement_duration_in_seconds": 15
}

When calibration_device is defined core begins with a so-called calibration phase, during which the user is asked to place a device (the calibration device) at a 1 meter distance from all boards, one at the time.
This phase aims to measure the average 1-meter-distance RSSI value so that the location estimation algorithm will use this value for each individual board.

[marcomicera]

Needs to be tested.

[marcomicera]

Nothing works when I try to merge this on master. I'll take care of this later.