fix(cros_sdk): Support using lbzip2 during chroot creation.

Our SDK tarballs aren't compressed using pbzip2 so there is no advantage
to using pbzip2 to decompress them over bzip2, however lbzip2 does offer
a big advantage. Also trust that the portage config defines a valid
version of bzip2 since we have control over the tarball creation and can
make sure to always include required utilities.
Merge pull request #62 from marineam/lbzip2

fix(cros_sdk): Support using lbzip2 during chroot creation.