• Find files that are CircuitPython-specific and change their headers to say they are part of the the CircuitPython project, not the MicroPython project. This helps distinguish upstream files that we use vs. files of our own. This change was discussed with the MicroPython folks. The new language is This file is part of the CircuitPython project: https://circuitpython.org.
• Change copyright and license lines to be SPDX-compliant. General format tested with reuse lint. There are still many files that are not SPDX-compliant, so we can't run reuse over the whole source tree.
• Remove in-line MIT license, since the license is now referenced by SPDX. The license can be found in licenses/.
• Add missing headers to a number of files. This was particularly common in boards/.
• Add headers to files that were previously completely empty.
• A few other small cleanups.
• The casual script I wrote to do this is in tools/circuitpy_header_change.py.
• This PR addresses primarily C files. There are files in other languages, text files, etc., that deserve these headers but don't yet have them.

I am not expecting the reviewers to go through every file! I will add reviewers after it builds successfully.

Changing header files to #pragma once will be another PR.