Let Debian install choose install disk by itself if none is given.

From http://www.debian.org/ :
# Alternatively, you may specify a disk to partition. If the system has
only
# one disk the installer will default to using that, but otherwise the
device
# name must be given in traditional, non-devfs format (so e.g. /dev/hda
or
# /dev/sda, and not e.g. /dev/discs/disc0/disc).
# For example, to use the first SCSI/SATA hard disk:
#d-i partman-auto/disk string /dev/sda
Merge branch 'partman-auto-disk' into devel

* partman-auto-disk:
  Let Debian install choose install disk by itself if none is given.