build: fixed SITL build flags
AP_AHRS: fixed functions that need to be virtual

functions overridden in a child class need to be marked virtual, or
you get the parent class function
autotest: fixed mavlink update path
RC_Channel: removed pwm_out variable from RC_Channel

saves 16 bytes
Rover: fixed SKID_STEER_IN calculations
autotest: check for new build tags
autotest: fixed rover mission completion
Revert "RC_Channel: removed pwm_out variable from RC_Channel"

This reverts commit 8e4a003d8dca3ee1bc3ae301d96c3cce53af40cf.

It is used by MotorsMatrix
Rover: minor parameter documentation updates
Plane: added HIL_SERVOS option

this allows for real servo output in HIL
Copter: add trapezoid frame type

We still need to properly define the motor mixing for the trapezoid
frame in AP_MotorsQuad.cpp
Add ability to compile sensor hil with sitl
Init hil accel with gravity
fix sitl segfault in hil
HIL fix location, alt, mag from hil state message
cleanup comments
Copter: ACRO bug fix from Jason
Copter: update Release notes ahead of 2.9.1b release
Copter: update Release notes again for 2.9.1b release

Missed item re increasing max inav accel correction to 3 m/s
Copter: update version to 2.9.1b-dev