build: add Raspberry Pi toolchain
build: add navio build target
AP_HAL: add configuration for HAL_BOARD_SUBTYPE_LINUX_NAVIO
AP_Baro_MS5611: add address switch for Navio board
HAL_Linux: Add GPIO driver for Raspberry Pi
HAL_Linux: Change SPI bus numbering to start from 0
HAL_Linux: Add SPI configuration for Navio
AP_InertialSensor: Use MPU9250 DRDY pin only on boards that support it
AP_Notify: Add dummy LED configuration for Navio