PX4: make FMUv2 identify itself differently 

so we can tell which is which in logs
AP_Compass: added more debugging to AP_Compass_PX4 driver

this helps debug compass initialisation issues
AP_InertialSensor_PX4: don't set QUEUEDEPTH

the default of 2 samples is fine
Rover: use L1 controller for navigation

this uses the AP_L1_Control library for rover navigation
Rover: don't use throttle_nudge in steering mode
Rover: send y accel in m/s/s as nav_pitch

this allows for easier tuning in MissionPlanner
SITL: reduce the minimum accel and gyro noise

on APM2 and PX4 we do a lot of smoothing of accels and gyros, so a 2
bit error isn't a good representation of the min noise
Rover: added STEERING_LEARN parameter

when enabled (set to 1) it will learn TURN_CIRCLE based on the
demanded and actual lateral acceleration
Rover: use Y accel computed from gyro and velocity

this should be less susceptible to noise and attitude errors,
hopefully leading to better learning
APM_Control: added a ground vehicle steering controller

this will be used both for the rover code, and for ground steering of
a plane on takeoff
APM_Control: fixed some documentation typos
AP_AHRS: added groundspeed() method

this gives ground speed in m/s from GPS
Rover: switched to new steering controller

this uses a steering rate controller, based upon the planes roll
controller
AP_Compass_PX4: removed MAGIOCSSAMPLERATE call