pico-sdk: Import picotool as a dependency

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>

config/data/pico-serprog/mkhelper.cfg

@@ -7,3 +7,4 @@ serx="$sersrc/build/pico_serprog.uf2"
 picosdk="src/pico-sdk"
 serdir="$picosdk/src/boards/include/boards"
 premake="mkserprog pico"
+picotool="$PWD/src/picotool/xbmkbin"