Many users report bugs, so I'm reverting lbmk back to only
supporting the rp2040 dongles for the time being. The
documentation will be updated to reflect this.

Pico2 support will be re-added at a later date, once more
testing has been done, and fixes made if necessary.

We were previously not handling picotool at all, and
pico-sdk would download picotool itself, at build time.

This means that the source archive, if created, would
not contain picotool. While not strictly required, for
complete corresponding source, since it's a toolchain
and not the actual pico-serprog firmware, it is my policy
that releases must include full corresponding source code,
when it is feasible to do so.

I must say, I intensely dislike cmake, with such burning
passion; I am thoroughly displeased by how hacky this is,
but it works and now nothing is in my way for a Libreboot
20241206 rev8 release!

Signed-off-by: Leah Rowe <leah@libreboot.org>