AP_Math: fixed example build
Rover: added BRAKING_SPEEDERR parameter

this controls how much speed error you need before brakes are used
AP_Mission: added get_next_ground_course_cd() API

this gives the ground course of the next navigation leg, which can be
used to calculate the amount of steering required
Rover: use the next navigation course to adjust steering gain in turns

this should make waypoints along a straight path not reduce speed