AP_Compass: ignore COMPASS_ORIENT for internal compass

this means if COMPASS_ORIENT is not 0 and the external compass fails
to start on boot we don't end up with a bad compass orientation
PX4: move PX4IO update to before any sensors start

this seems to make the update much more reliable, possibly as it
avoids an interrupt load from l3gd20 driver
AP_InertialSensor: fixed example build
GCS_MAVLink: added FENCE_ACTION_GUIDED_THR_PASS
Plane: added FENCE_ACTION=3 behaviour

this makes it safer to enable the geofence as a receiver failsafe
mode. It retains manual throttle control, so if it triggers on the
ground due to the receiver losing signal from the transmitter then the
throttle can be kept low, preventing an unexpected takeoff.
Copter: remove unused get_acro_pitch and roll
Copter: add #include for AC_Sprayer
Copter: move init of global nav variables to set_nav_mode
Copter: remove manual_attitude and throttle flags
Copter: move acro's var initialisation to roll-pitch and yaw controller initialisation
Copter: add arm check of GPS hdop when in Loiter
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