unmanned ground vehicle (UGV) controllers and Traxxas Stampede configuration (#7175)

ROMFS/px4fmu_common/init.d/50002_traxxas_stampede_2wd

ROMFS/px4fmu_common/init.d/rc.axialracing_ax10_defaults

ROMFS/px4fmu_common/init.d/rc.gnd_apps

ROMFS/px4fmu_common/init.d/rc.gnd_defaults

ROMFS/px4fmu_common/init.d/rcS

ROMFS/px4fmu_common/mixers/stampede.main.mix

ROMFS/sitl/mixers/rover_sitl.main.mix

cmake/configs/nuttx_aerocore2_default.cmake

cmake/configs/nuttx_aerofc-v1_default.cmake

cmake/configs/nuttx_auav-x21_default.cmake

cmake/configs/nuttx_mindpx-v2_default.cmake

cmake/configs/nuttx_px4fmu-v2_default.cmake

cmake/configs/nuttx_px4fmu-v3_default.cmake

cmake/configs/nuttx_px4fmu-v4_default.cmake

cmake/configs/nuttx_px4fmu-v4pro_default.cmake

cmake/configs/nuttx_px4fmu-v5_default.cmake

cmake/configs/posix_rpi_common.cmake

cmake/configs/posix_sitl_default.cmake

posix-configs/SITL/init/ekf2/rover

posix-configs/SITL/init/lpe/rover