Switch the portage tarball from xz to bz2

Docker 1.3.2 currently has a bug wrt using xz from a sandbox. See
https://github.com/docker/docker/issues/9327

We loose ~13% bandwidth but at least have a working build
Merge pull request #14 from jbergstroem/docker-1.3.2-xz

Switch the portage tarball from xz to bz2