Generalizing support for UAVCAN-interfaced sensor nodes

Usage:
```bash
# Activate the appropriate UAVCAN <--> ORB sensor bridge:
uavcan sensor enable <sensor name>

# List all bridges enabled at the moment:
uavcan sensor list

# Print all valid bridge names with general usage info:
uavcan
```

Supported sensors:

* GNSS
* Magnetometer
* Barometer

This PR is not ready for merge yet because the newly implemented UAVCAN sensor support logic lacks integration with rc scripts.