Add support for timesync from onboard companion computer

This is a part of building in support for timing and sync of messages coming from an onboard companion computer.  While this could be done more elaborately, I think this should suffice. 
I will add more support to keep track of computer's clock to align time, so that it is useful for logging, stamped vision messages, etc. Would it be better to add a uORB topic for time and then add another module for maintaining time all over the system, tracking clock of companion computer and adding offsets, etc.?

sdlog2 I think needs to be modified to use system time, instead of time directly from vehicle_gps_position topic.
This should probably not be static, but per MAVLink instance. You can have two computers / one computer, one GCS talking to the system and we want to keep track of each their offset.