start seperating swd multi drop

[perigoso]

Detailed description

I initially meant to properly abstract away the multidrop handling and swd scan in general, but left it rather early, this PR doesnt have much going for it, just a place to document and later continue work from

as is the jlink scan (and I believe stlink too, but i have not tested or looked at it) does not handle multi-drop, I was meaning to get rp2040 working on jlink as means to develop and test this work but it is unable to comunicate with its DP, the reason is I would guess the Lack of "selection alert sequence for SW-DP-V2" that is present on the regular adiv5_scan

The jlink probe and api should be able to do it, but it needs to be implemented, ideally as a seq_out method, and the cmds in jlink in general could do with some work

since the jlink usb spec is not that easy to find I will leave it here
RM08001_JLinkUSBProtocol_Rev.7.pdf

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)
• It builds as BMDA (make PROBE_HOST=hosted)
• 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]

It would be useful please if this PR could be rebased on main, especially now that the J-Link implementation implements those low-level SWD functions mentioned as being a blocker for you.

Having proper support for multi-drop and it being refactored like this is of great interest to us

[perigoso]

It would be useful please if this PR could be rebased on main, especially now that the J-Link implementation implements those low-level SWD functions mentioned as being a blocker for you.

Having proper support for multi-drop and it being refactored like this is of great interest to us

Thank you for letting me know, this was just the start of the implementation there's work to do still, but I was looking for something to pick up!

[perigoso]

Unsure if ready to merge

The context around this PR has changed quite a bit over the time it sat idle, one of the main objectives (J-Link multidrop) was solved in the meantime.
For now this just provides some refactoring and code separation for multi-drop, with some code size saving in the mix.

Note to self, look at targetsel handling correctness and generalization

[perigoso]

After taking a new look, this is ready for review @dragonmux it doesn't achieve much worthy of note other than code size (and readability I guess), but, stepping stones...

I started to poke the targetsel handling, but I only got higher not lower flash usage and the code didn't improve significantly, so I'm leaving that for another time.

[dragonmux]

This is looking great and provides some much needed clean-up and magic number naming - thank you for the contribution!

There are a few items that we've spotted in review that could do with addressing before we merge to try to ensure this is the best possible version of a SWD bus scanner, but with them addressed, we'll be happy to merge this.

[dragonmux]

LGTM, thank you for the contribution! Merging.

We'd run our own tests but we know you're careful to thoroughly test these kinds of thing.