Do not depend on dpkg-divert being available inside /usr/sbin

Upload of dpkg 1.17.0 included the following change:

|  * Remove update-alternatives, dpkg-divert and dpkg-statoverride
|    compatibility symlinks under /usr/sbin/.

So we can't rely on dpkg-divert being available inside /usr/sbin.
For example systems diverting /sbin/init might end up with boot
failure:

| switch_root: cant execute /sbin/init: Exec format error

because the dpkg-divertion doesn't get restored.

Acked-by: Thomas Lange <lange@informatik.uni-koeln.de>