Add support for producing UF2-format binaries.

[tzarc]

Description

This PR adds support for producing UF2-formatted binaries using the utility scripts provided by the microsoft/uf2 repository.

Raspberry Pi's RP2040 has in-silicon bootloader UF2 support for flashing binaries. There are other bootloaders available that do the same thing for other MCU families -- see https://github.com/microsoft/uf2#family-list -- so I'm adding this early so that it's locked in by the time ChibiOS picks up RP2040 support.

Flashing is not currently implemented -- the same conversion script can handle deployment -- this will follow later.

Types of Changes

• Core
• Bugfix
• New feature
• Enhancement/optimization
• Keyboard (addition or update)
• Keymap/layout/userspace (addition or update)
• Documentation

Checklist

• My code follows the code style of this project: C, Python
• I have read the PR Checklist document and have made the appropriate changes.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• I have tested the changes and verified that they work and don't break anything (as well as I can manage).

[fauxpark]

Another submodule? 😢

[elfmimi]

Another submodule? 😢

It's a pretty simple single file script.
https://github.com/microsoft/uf2/blob/master/utils/uf2conv.py
so I suppose it is a reasonable choice to copy this one into the repo as util/uf2conv.py .
※ ideally it should be available as a python package, though.

[tzarc]

※ ideally it should be available as a python package, though.

Ideally... but as far as I can see it isn't :(

Anyways, copied to util/uf2conv.py.