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
PX4: start px4flow driver
Copter: enable OPTFLOW - do not merge to master
Copter: optflow parameters moved to optflow class
Copter: simpler optflow logging

Removed of_loiter control output from message
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
Copter: OF log fix to save vel as float
Copter: add TimeMS to OF dataflash log msg
OptFlow: add ground_distance_m
OptFlowPX4: retrieve ground distance from sensor
Copter: log ground distance from optflow sensor
AP_NavEKF : Added additional states required for optical flow

This adds a terrain position state to the main filter, and also creates a single state EKF
for estimation of focal length scale factor.

TODO interface the ReadFlowData functon with the optical flow sensor library
AP_NavEKF : Use averaged angular rate for flow motion compensation
AP_NavEKF : Fusion of motion compensated optical flow rates into main filter
Not For Master : quick hack to remove compiler errors for testing