exclude sysvinit/ sysvinit-core and include systemd-sysv for cdebootstrap.

* exclude sysvinit/ sysvinit-core and include systemd-sysv for cdebootstrap.
* truncate /etc/machine-id to zero byte instead of excluding it. A zero-byte
  file is handled specially by systemd and regenerates a fresh machine-id
  while booting.