GCS_MAVLink: added have_flow_control() method

can be used to change speed of handling some protocol methods, as we
know communication will be reliable
Plane: much faster mission upload

send next WP request immediately if possible. This speeds up mission
upload on USB by about 10x
Plane: use _send_buf() functions to reduce stack usage in MAVLink replies

this re-uses the incoming message buffer in constructing the reply
GCS_MAVLink: re-generate MAVLink headers
GCS_MAVLink: moved parameter and stream handling into common code

this keeps it in common between vehicles, and saves stack space
GCS_MAVLink: moved RADIO_STATUS handling into common code
GCS_MAVLink: added save parameter to handle_request_data_stream()

this allows copter to not save stream rate changes
GCS_MAVLink: make DataFlash a pointer

handle sketches where DataFlash is not available
Plane: use new common MAVLink code
Copter: use new common MAVLink message handlers
Rover: use new common MAVLink message handlers
AntennaTracker: fixed for new MAVLink handling
CPUInfo: fixed build
VARTest: fixed build
AP_InertialSensor: fixed example build
AP_Math: fixed example build
AP_Mount: fixed example build
AP_Notify: fixed example build
DataFlash: fixed example build
GCS_Console: fixed example build