'Untar the package present in the tarball_path and put a
".checksum" file in the dest_dir containing the checksum
of the tarball. This method
assumes that the package to be untarred is of the form
<name>.tar.bz2'

def untar_pkg(self, tarball_path, dest_dir):

    os_dep.command('tar')
    self._run_command(('tar    xjf    %s    -C    %s' % (tarball_path, dest_dir)))
    pkg_checksum = self.compute_checksum(tarball_path)
    pkg_checksum_path = os.path.join(dest_dir, '.checksum')
    self._run_command(('echo    "%s"    >    %s    ' % (pkg_checksum, pkg_checksum_path)))
