TriCopter: support servo with angle feedback to remove need to set YAW_SV_ANGLE

#### Issue details

_Please describe the problem, or desired feature_
Would like to make a request to consider adding dynamic yaw input from the yaw servo in a tricopter. Triflight ( https://github.com/lkaino/Triflight/releases ) uses this feed back data. Also RCExplorer.de sells a modified Blue Bird 210DMS servo with a feedback lead. http://rcexplorer.se/product/blue-bird-bms-210dmh-servo/ It uses a magnetic induction rotary encoder to know the rotor angle which can be used in calculations. This seems like a good idea. Please forgive me if its stupid or already address. I am willing to test.
#### Version

_What version was the issue encountered with_
#### Platform

[ ] All
[ ] AntennaTracker
[ x ] Copter
[ ] Plane
[ ] Rover
#### Airframe type

_What type of airframe (flying wing, glider, hex, Y6, octa etc)_
#### Hardware type

_What autopilot hardware was used? (pixhawk, pixracer, PX4FMU etc)_
#### Logs

_Please provide a link to any relevant logs that show the issue_

The issue names no files, tests, or entry points. Start by locating the tricopter yaw-servo handling and reviewing the angle-feedback hardware described in the request; done would require a defined implementation scope and validation criteria for dynamic yaw input.

Written by the indexing model from the issue text.