Generalizing support for UAVCAN-interfaced sensor nodes

Usage:
```bash
uavcan start
```

Config params:

* `UAVCAN_ENABLE` (default: 0)
* `UAVCAN_NODE_ID` (default: 1)
* `UAVCAN_BITRATE` (default: 1M)

Supported sensors:

* GNSS
* Magnetometer
* Barometer