AP_InertialSensor: allow for 3 accels and 3 gyros on Pixhawk

this supports FMUv3
DataFlash: allow logging of 3 accels/gyros
PX4: startup FMUv3 sensors if available
DataFlash: save some flash space on APM2

don't include log message headers that are not used on APM2
CameraFeedback: added CAMERA_FEEDBACK_FLAGS for open/closed loop

This is so a GCS can tell if a certain picture was definitely captured
(e.g. with a CCB or machine vision cam) or only requested in the blink
(e.g. uni-directional CHDK)
GCS_MAVLink: regenerate mavlink headers
AP_Math: Comments on WGS coordinate conversions
AP_NOTIFY: RTK GPS visual notification through faster blink
SITL: Piksi Heartbeat Message support, Integer-RTK baseline simulation
AP_GPS: SwiftNav RTK Driver and GPS AutoSwitch param
ArduCopter: GPS Callback for calculating base position
MAVLink: New message definitions for GPS_RTK
GCS_MAVLink: Add support for GPS_RTK messages
AP_GPS: GPS calculates MAVLink messages for GPS and GPS_RTK
ArduCopter: Move MAVLink size check into library
GCS_MAVLink: regenerate headers
GCS_MAVLink: fixed flymaple build

has dual GPS but no RTK support
AP_GPS: fixed build errors and warnings in SBP driver

shadowed variables and implied casts in structures
GCS_MAVLink: added missing RTK headers