Improvements to SITL to make paths more flexible. (#5181)

CMakeLists.txt

Makefile

Tools/sitl_gazebo

Tools/sitl_run.sh

cmake/common/px4_base.cmake

cmake/configs/nuttx_mindpx-v2_default.cmake

cmake/configs/nuttx_px4-stm32f4discovery_default.cmake

cmake/configs/nuttx_px4fmu-v1_default.cmake

cmake/configs/nuttx_px4fmu-v2_default.cmake

cmake/configs/nuttx_px4fmu-v2_ekf2.cmake

cmake/configs/nuttx_px4fmu-v2_test.cmake

cmake/configs/nuttx_px4fmu-v4_default.cmake

cmake/configs/nuttx_tap-v1_default.cmake

cmake/configs/posix_bebop_default.cmake

cmake/configs/posix_eagle_default.cmake

cmake/configs/posix_eagle_hil.cmake

cmake/configs/posix_eagle_legacy_driver_default.cmake

cmake/configs/posix_eagle_muorb.cmake

cmake/configs/posix_rpi_cross.cmake

cmake/configs/posix_rpi_native.cmake