Remove Git submodules

Use CMake's FetchContent feature instead, the same as is already done
for GoogleTest.

This change necessitates copying flatc and reflection.fbs as part of the build process to a known
location in the source directory, since the flatbuffer sources are no longer directly in the source
tree. CMake targets have been added to do this. regenerate_headers.sh is updated accordingly.

Signed-off-by: Michael Platings <michael.platings@arm.com>
Change-Id: Idc08297a3e8dac3da978e5d537b1e6d04279a070