HAL_PX4: support uartE for 2nd GPS
AP_HAL: support uartE
HAL_AVR: no uartE
HAL_Empty: no uartE
HAL_FLYMAPLE: no uartE
HAL_SITL: support a 2nd GPS
DataFlash: added GPS2 logging
Plane: support a 2nd GPS
Rover: support a 2nd GPS
Copter: added support for 2nd GPS
AP_Motors: removed unused variable
AP_AHRS: remove unused variable