The old chroot version system we inherited from Chromium OS always
assumes that a newly unpacked tarball is the latest and greatest but
since we version the SDK in the same way as target builds we can use
that version for these sorts of upgrade scripts and not make assumptions
about how late and great the starting tarball was.
The first upgrade script simply aborts to force the user to recreate
their chroot when moving from python 2.6 to 2.7.
