Copter: move update_optflow to sensors.pde

No functional change
Copter: optflow instantiation format change

No functional change
Copter: add OPTFLOW to default logging
Copter: optflow logging minor format change
Copter: send optflow health in extended status
Plane: send optflow health in extended status
Copter: remove unused optflow roll, pitch variables
GCS_MAVLink: add OPTICAL_FLOW to ap_message enum
Copter: send OPTICAL_FLOW msg to GCS
Plane: send OPTICAL_FLOW msg to GCS