build: fixed build_all_px4.sh for new PX4 build
Copter: only run nav controllers when auto-armed

This stops run-up in target position and nav controller I terms ahead of
throttle being raised
Copter: remove redundant init of controllers when auto is started

individual do_* commands are responsible for setting roll-pitch, yaw and
throttle modes.  Removing this redundant setting of the controllers
removes the short period (10th second max) where loiter and waypoint
controller's desired roll and pitch were used even before the
controllers have been run once.
Copter: set yaw-mode in do_loiter mission command
Copter: rename verify_must to verify_nav_command

Perhaps easier to understand if we use the "nav" and "conditional"
command labels
Copter: allow lat/lon to be specified for land
Compass: update parameter description

We do not want people modifying the COMPASS_MOTCT manually
WPNav: make get_stopping_point method public
Copter: bug fix for RTL bearing

Also smooth RTL's initial climb stage by projecting stopping point
MAVLink: update to latest common.xml from upstream
PX4: don't start px4io twice
Copter: bug fix for slow loiter repositioning
AP_L1_Control: Remove potential nan errors

If WP A and B were the same or ground speed was exactly zero, then the
previous code would produce a nan output.  Protection against these
two cases has been added.

If WP A and B are equal, we track directly to the target waypoint
AP_AHRS: use vectors for ground vector complimentary filter
PX4: fixed handling of nostart option
AP_HAL: added voltage_average_ratiometric() call to AnalogIn

this is for ratiometric sensors such as the 3DR airspeed sensor and
the Maxbotix analog sonar
AP_Airspeed: use rationmetric analog input for airspeed sensor
AP_Rangefinder: use ratiometric analog input for analog sonar
AP_ADC_AnalogSource: added voltage_average_ratiometric()
Rover: use voltage_average() for RSSI

this allows it to work on PX4