HAL_PX4: implement analog input stop pins

this allows multiple sonars to cooperate without interference
AP_HAL: added HAL_ prefix to GPIO_ defines

this fixes a bug caused by GPIO_INPUT and GPIO_OUTPUT already being
defined in NuttX, which caused pinMode() not to setup pins for output
when requested
HAL_AVR: fixes for HAL_GPIO_ define change
HAL_FLYMAPLE: fix for HAL_GPIO_*
AP_InertialSensor: fixes for HAL_GPIO_*
AP_Notify: fixes for HAL_GPIO_*
AP_Relay: fix for HAL_GPIO_*
DataFlash: fix for HAL_GPIO_*
Rover: fix for HAL_GPIO_*
Plane: fix for HAL_GPIO_*
APM_OBC: fix for HAL_GPIO_*
AP_AHRS: fix for HAL_GPIO_*
AP_Baro: fix for HAL_GPIO_*
AP_EPM: fix for HAL_GPIO_*
AP_Compass: fixed VRBrain build
APM_OBC: fixed formatting to match APM coding standard