[Toybox] Moved mkroot to its own directory.

[Rob Landley]

Heads up, scripts/mkroot.sh is now mkroot/mkroot.sh. If you run the old name it
says it moved.

I also moved testroot.sh, record-commands, and the mkroot/root directory, and
I'm working on a README (kinda sort based on the old standalone
https://github.com/landley/mkroot/blob/master/README and
https://landley.net/toybox/faq.html#mkroot but... it's being stroppy).

I haven't moved mcm-buildall.sh (yet?) because I need to rewrite it not to use
Rich Felker's musl-cross-make. (Which he hasn't pushed a commit to in over a
year. I should probably call it cccbuild.sh or some such, _and_ eventually teach
it to do musl llvm toolchains too, and maybe bionic? But I have some shoveling
through my todo list to get down that far...)

Rob