Bus and sensor health indication

#### Issue details
Need a means to indicate bus health and sensor health during bring up of a vehicle. Currently, there is no way for the system to help vehicle builders find miswires and other issues, aside from inspection of the wiring against pinouts. This leads to a lot of wasted time looking at configuration settings and other bugs.

This issue was referred from the Partners Call in April 2018.

@tridge proposed adding mavlink messages to support more detail health status information.

Then GCS tools can pick up these messages and display them in a useful way.

#### Version
copter 3.5.5 is the most recent place I've had this issue, but I've also had the issue on rover.
#### Platform
[ x ] All
[ ] AntennaTracker
[ ] Copter
[ ] Plane
[ ] Rover
[ ] Submarine

#### Hardware type
All

#### Logs
No logs on this currently.

Start by defining the bus and sensor health information that vehicle builders need during bring-up, then review the proposed MAVLink message approach and how GCS tools would consume it. Done means health status can be reported for the affected systems and displayed usefully by GCS tools; the issue names no implementation files or tests.

Written by the indexing model from the issue text.