Fix: Remote protocol packet handling organisation

[dragonmux]

Detailed description

In the current implementation of the GDB remote protocol in BMD, the code for processing incoming packets and dispatching based on them being actual GDB remote protocol packets or BMD remote protocol packets is kind of a mess - spaghetti code even.

This PR addresses this with a reorganisation that helps simplify the code flow and reduce cognitive complexity. This forms the basis of part of the work of PR #1361 and bringing this oft overlooked part of the code base up to standard.

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

[esden]

Damn that was bothering me forever. Thanks for reorganizing this code.