* Lists ROMFS files explicity instead of using GLOB_RECURSE

Previously, when ROMFS files that were not airframes were touched, the ROMFS
would not be rebuilt. The ROMFS files are now specified explicityl in a
CMakeLists.txt file that is located in the root ROMFS directory.

Now when one of the ROMFS files is touched the whole ROMFS is rebuilt.

When new files are added to the ROMFS, they need to be explicity added to
the CMakeLists in the ROMFS root directory.

* ROMFS: adds individual CMakeLists files in each subdirectory

Also moves the temporary ROMFS build directory to ${PX4_BINARY_DIR}/ROMFS/genromfs
so that the cmake_install.cmake files and the CMakeFiles directories (generated whenever
are not add_subdirectory() is called) are not generated in the temporary ROMFS directory
from which the ROMFS binary is created.

* cmake ROMFS generate add px4_add_romfs_files function

* ROMFS CMakeLists: adds explanatory comment to px4_add_romfs_files function

* ROMFS CMakeLists: updates copyright headers