HAL_PX4: fixes for merged upstream PX4Firmware
DataFlash: fixed a build warning
AP_HAL: removed RCInput valid_channels() and added new_input() and num_channels()

the valid_channels() method was inconsistently implemented between
boards, and served two quite different purposes. It is clearer as two
functions
Rover: updates for new RCInput API
Copter: updates for new RCInput API
Plane: updates for new RCInput API
Rover: support 14 RC channels on PX4
Copter: support 14 RC channels on PX4
Plane: support 14 RC channels on PX4
GCS_MAVLink: merge latest msgs from upstream
GCS_MAVLink: rebuild MAVLink headers
Plane: report up to 18 RC input channels via MAVLink
Rover: support 18 input channels in MAVLink
Copter: support 18 input channels in MAVLink
RC_Channel: support up to 12 aux channels on PX4

this allows RC13 and RC14 to work correctly as aux outputs on Pixhawk
DataFlash: support logging up to 14 RC input channels