Extend minimal native build environment into a seed for Gentoo Catalyst.

This doesn't quite create an official Gentoo Stage 1.  We use busybox instead
of gnu tools, we're uClibc-based instead of glibc-based, and we use our
existing toolchain (with distcc acceleration) instead of asking portage
to build one.  That said, this should be enough to run Catalyst and produce
official Stage 1, Stage 2, and Stage 3 images.

GFS used:

  setup-base-packages.sh
    strace, Python, ncurses, bash, tar, patch, findutils, file, pax-utils,
    shadow
  setup-portage.sh
    /etc/passwd (root and portage), /etc/group (root and portage)
    portage