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