Build: enable px4flow driver on Pixhawk
OptFlow: rename and restructure OpticalFlow class

AP_OpticalFlow.h becomes simply a file that includes all other optical
flow header files.
OpticalFlow class simplified to only return surface quality, raw output
and velocity vector.
OptFlow: reorganise ADNS3080 to simplified interface
OptFlow: add PX4Flow support
Copter: optflow parameters moved to optflow class
Copter: simpler optflow logging

Removed of_loiter control output from message
add TimeMS to OF dataflash log msg
Copter: instantiate optflow on Pixhawk

Run sensor reads from scheduler
Copter: integrate optflow_enable param move to optflow class
Copter: Of_Loiter uses sensor velocity instead of integrated position
OptFlow: add ground_distance_m
Copter: log ground distance from optflow sensor
OptFlowPX4: use ORB to pull data from sensor

This change is required because PX4Firmware has changed the method used
to pull data from the sensor.
Copter: pilot override used immediately in OF_Loiter

Replace some hard coded numbers with definitions
Copter: enable optflow by default for Pixhawk
OptFlowPX4: fix compile error for APM
Copter: move update_optflow to sensors.pde

Also slightly shorten function name
OptFlow_ADNS3080: check healthy before updating
OptFlow_PX4: check healthy before updating
Copter: disable OptFlow in SITL
HAL_AVR: fixed warning