[Toybox] I merged mkroot.

[Rob Landley]

So if you "make root" it'll build a simple toybox based root filesystem, and if
you have $LINUX pointing to a linux source directory, it'll build it based on
whatever architecture your cross compiler points to.

Right now it's detecting architecture for the kernel build from the cross
compiler prefix (so it works for the musl-cross-make compiler names), I might
instead run file against the toybox binary and try to parse the output, but
that's not in yet.

Also, right now you need to switch on "sh" and "route" in pending to get
anything useful, and sh isn't really ready for prime time yet. But this is more
or less what it should look like. :)

Rob