In AP_MotorsTri.cpp the AP_MOTORS_MOT_1, _2 and _4 constants are
always mapped to actual output channels through _motor_to_channel_map
while the _CH_TRI_YAW is not, but there were a few inconsistencies
in this that could lead to things like PWM min and max values being
set on wrong channels.
It looks like all in all _motor_to_channel_map being in PROGMEM
probably doesn't help save memory and I'm not sure how useful it is
in the first place but regardless the usage should be consistent.
1. it's step towards rearchitecting the current code base ahead of a move to RTOS.
2. internally it uses a MatrixTable for the Quad, Octa, OctaQuad, Y6 frames.
3. it implements the missing stability patch for Octa and OctaQuads (still missing for Y6)
Later check-ins will incorporate into the main ArduCopter code.