Univention Corporate Server 4.1 erratum 116 (bugfix)

This update addresses the following issue(s):
On UCS systems booting via BIOS, an error would happen if debconf
grub-pc/install_devices contains a wrong device.
If it contains a wrong device the GRUB installation happens but
fails, leading to an inconsistent installation between /boot/grub and
the GRUB directly on the disk.
This makes the system unbootable.

This update checks all devices in grub-pc/install_devices, removing
invalid devices.

Additionally a guess is made for the correct boot device which will
be added to grub-pc/install_devices if grub-pc/install_devices is
currently empty or there were invalid devices.

If any changes were made, grub-install is run on all devices in
grub-pc/install_devices.

See also <http://sdb.univention.de/1356> and
<http://sdb.univention.de/1357>.