GPS link delay

[olwyk]

Calculate link delay due to processing time between nodes using GPS timestamps.

[JMare]

The best way to do this would be to use GPSD to handle the gps hardware and then query it with cmavnode.
Or we could just take the system time as true and leave it up to the user to make sure the times are synced up (RTC + NTP should give us some level of accuracy?)