Include Frozen Libraries In build_board_info.py

[sommersoft]

Fixes #4672

Tested with the following:

(.venv) ~/Dev/circuitpython/tools:$> python
Python 3.8.2 (default, May 20 2020, 16:07:42) 
[GCC 7.5.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import json
>>> import pathlib
>>> from build_board_info import get_board_mapping
>>> fpath = pathlib.Path().home()
>>> with open(fpath / "Dev" / "frozen_board_mapping.txt", "w") as f:
...     json.dump(get_board_mapping(), f, indent=2)
... 
>>>

Result:
frozen_board_mapping.txt

Note: without a manual update, this will only update the newest version in circuitpython-org/_data/files.json.

[tannewt]

Thanks @sommersoft !