feat(common): Add simple version comparison function.

Relies on sort for version comparison which should generally be good
enough. Not a proper semver tool but good enough for dirty scripts. :)
fix(update_chroot): Replace old chroot version scheme.

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.
fix(make_chroot): Write sudoers.d/90_cros directly.

Previously this called a file in the now deleted chroot_version_hooks.d
which surprised me to say the least but it really shouldn't have...
fix(make_chroot): Remove the old "force-to-latest" chroot version step.

This script is gone and dead.
fix(update_chroot): Remove an old debug echo
Merge branch 'master' of ssh://github.com/coreos/scripts