Misc fixes

[9ary]

This PR supersedes #105. It includes misc cleanups, fixes two implementation bugs, and decouples the payload from picoboot.

Fixes #22 Fixes #58

[9ary]

Regarding the last commit (merging the payload into a unified UF2 file), I'm not sure I actually want to keep this:

• gekkoboot is not the only payload (Swiss or cubeboot are also good options)
• I'd prefer to make users download payloads from their upstream repos instead of bundling them here:
  • it ensures users always download the latest version of the payload without having to update it here
  • it makes it clearer that payloads are separate projects, rather than part of PicoBoot; I've seen a lot of people being confused about this on discord

My preferred solution would be to bundle a default no-op payload that shows a message directing the user to the install instructions.

Modxo (Xbox modchip) uses a separate UF2 for the payload as well. They have a web-based converter to pack Xbox BIOSes into the correct format. Doing that for arbitrary DOLs would require a bit of work because of the entry point, but it's feasible. https://github.com/shalxmva/modxo?tab=readme-ov-file#flashing-steps

[webhdx]

Thanks. Everything looks good to me now.