Feature: Space-efficient 12-symbol serialno generation

[ALTracer]

• This can be considered a new feature (because I don't see how it was usable previously)
• There is an existing problem of BMPBootloader with DFU_SERIAL_LENGTH=13 becoming larger than 16 KiB due to siprintf pulled from newlib (not nano).
• The PR solves this problem by replacing the single expensive sprintf() call with a few calls of simpler functions and introducing a variant of utoa().

12-symbol serialno consists of three halfwords calculated by pulling 16-bit values from Device Electronic Signature memory. I don't believe hexify() can be useful here.

I would not like to choose to mitigate this by downgrading the F4 BMPBootloader to newlib-nano because it would provide a small and inefficient memcpy, which is important for flash writes/upgrade reprogramming speed. And it's not like a bootloader would use *printf anyway.

Your checklist for this pull request

• I've read the Code of Conduct
• I've read the guidelines for contributing to this repository
• It builds for hardware native (make PROBE_HOST=native) -- and is not applicable
• It builds as BMDA (make PROBE_HOST=hosted) -- and is not applicable
• I've tested it to the best of my ability
• My commit messages provide a useful short description of what the commits do

Closing issues

[dragonmux]

We like the idea of this PR, the execution needs some love though - please see our review notes below.

[dragonmux]

Most of the review items have been addressed and this is looking better for it - there are a couple of things we've spotted this run through and think once those are addressed this should be about ready for merge if you could squash commits appropriately.

[dragonmux]

This all looks good to us now - we're happy to merge this once the copyright and authorship notice issue is fixed as noted on Discord.

[ALTracer]

Squashed intermediate progress into two distinct commits: one adding the function, one adjusting the callsite to use it (so no builds fail). Added a separate commit to refresh copyright info on the file.

Thank you for propelling this PR with your extensive review. We're one step closer to making BMPBootloader/F4 DFU nicer to use (and went through the practice of adding a helper function).

[dragonmux]

Merging, thank you for the contribution!