AP_HAL: support uartD on some boards

PX4 and SITL get an extra UART
HAL_AVR: added NULL uartD
HAL_SITL: added uartD
HAL_EMPTY: added uartD
HAL_FLYMAPLE: added uartD
HAL_LINUX: added uartD
HAL_PX4: added uartD
GCS_MAVLink: added support for 3 UARTs

support 3 UARTs on non-AVR platforms
Plane: added 3rd telemetry stream on platforms that have it

this generalises the gcs object usage
PX4: setup uartD on PX4

on FMUv1 enable uartD only if /fs/microsd/APM/uartD.en exists

on FMU2 always enable uartD
Plane: fixed mavlink_queue[] size
AP_AHRS: added attitude_reset() method for HIL_SENSORS
Plane: cope with uartD being NULL
AP_AHRS: fixed a build warning
Copter: added uartD support

this also brings GCS_Mavlink.pde closer to the plane implementation