generate_download_info(): teach it about the language allow list

In #4298 we added an allow list of languages to be built in the github CI.  However, this allow list was not taken into account in generate_download_info (inside tools/build_board_info.py).

Move the allow list from tools/build_release_files.py to build_board_info.py, and use it from both places.

While you're in there check if you can use json.write / json.dumps with sort_keys=True 
