cmake handle git submodule depdencies

CMakeLists.txt

Tools/check_submodules.sh

cmake/common/px4_git.cmake

platforms/nuttx/NuttX/CMakeLists.txt

src/drivers/gps/CMakeLists.txt

src/firmware/posix/CMakeLists.txt

src/firmware/posix/sitl_target.cmake

src/modules/ekf2/CMakeLists.txt

src/modules/mavlink/CMakeLists.txt

src/modules/simulator/CMakeLists.txt

src/modules/uavcan/CMakeLists.txt