Fix disabling framebuffer use in d-i

  * debian/brltty-udeb.init: Remove udevadm settle call, and move disabling
    framebuffer to...
  * debian/brltty-udeb.sh: here, but create a
    /lib/debian-installer.d/S20brltty script instead of doing it immediately,
    since debconf might be currently running and about to overwrite the
    database. Last but not least, kill debconf to let it restart without
    framebuffer (Closes: #611648).