APM_Control: Pitch Control - Reduced the speed below which the integrator is locked

This is to allow for slow speed flare manoeuvres in FBW modes
APM_Control: Added integrator limiting adjustable by an advanced user parameter

this is compatible with the old IMAX settings
Plane: Added APM_Control integrator limits to table used to covert old PID values
Copter: minor param definition unit changes
PX4: use PX4_ROOT=../PX4Firmware by default

this should make PX4 builds work out of the box on Windows for people who have used
the github for windows tool to download the two repositories
Plane: APM2 doesn't have channel 9 PWM output
autotest: consider lack of symlinks to be a soft error

some people use VMWare shared drives, which don't do hard links
PX4: added ARDUPILOT_BUILD define

will be used to avoid some code in the px4io driver
PX4: log devices being used in boot.log
AP_Relay: added RELAY_PIN parameter

makes it easier to choose a pin
AP_Camera: added CAM_TRIGG_DIST parameter

for triggering every N meters of GPS distance travelled
Plane: enabled RELAY_* parameters
Plane: enable CAM_TRIGG_DIST parameter
Plane: update_mount assumes 50Hz, for camera trigger
AP_AHRS: cope better with large GPS yaw changes

this should cope better with bungee launches when using only GPS for
yaw.
Copter: enable RELAY_* parameters
Copter: CONFIG_RELAY is not used
Rover: enable RELAY_* parameters
Copter: enabled CAM_TRIGG_DIST parameter
AP_InertialSensor: relax accel offset sanity check