Fix update instructions to use picotool instead of elf2uf2-rs

[trappitsch]

Fixes the instructions for building/flashing new firmware using picotool instead of elf2uf2-rs.

Also update .cargo/config.toml to provide a picotool load... runner.

No change here on the "Running with defmt logs without debugger" section, which also seems to be using elf2uf2-rs and I don't think is working either.

[Yatekii]

Why is elf2uf2-rs not working anymore?

[trappitsch]

Sorry, should have given more content here. elf2uf2-rs has been pretty flaky, see, e.g., this and this discussion. There were also various messages in the rp-rs and Embassy matrix rooms on this. The general recommendation recently has been to use picotool. There is also elf2flash that fixes some of the issues, see discussion at the bottom of their readme.

That being said: I just tried to reproduce the error message I got this morning with elf2uf2-rs... but after installing it again to reproduce, I don't see the error anymore and it makes a uf2 file. This might be my day-of-outdated-versions...

Sorry, this should have been an issue to discuss first an not a PR, feel free to close, tell me to summarize in issue, or discuss further.

In case of the latter, here's an idea / proposition:

• put a note/warning on elf2uf2-rs issues in the readme that states the current potential issues users might run into
• provide instruction to use picotool as an alternative build way to generate the firmware

Again, apologies for an over-hastily prepped PR.