Remove cross-platform-software

cross-platform-software's readme clearly states that "it contains functionality that is used across subsystems that use ATSAMs." And realistically, its main branch has diverged so much from ours in ways which are mostly irrelevant to COMMS that there is no point in keeping it as a submodule. On the contrary, we can merely integrate the used files on our repo.

Moreover, such a simplification of our codebase could help the infamous conan integration. See #63 and !30 (closed). (Yes, we will have to do that eventually...)