Workaround for issue #4016.
Workaround for issue #4016.

In this commit, we add a step in the prepare script to generate
`/etc/machine-id` before the systemd package is configured in
`debootstrap --second-stage`. This basically makes
`systemd-machine-id-setup` a no-op when it is invoked as part of
systemd's postinst script, thus avoiding the problem in issue #4016.

dnschneid/crouton,dnschneid/crouton,dnschneid/crouton,dnschneid/crouton,dnschneid/crouton,dnschneid/crouton