Removed useless update-grub run in preliminary image.
Install linux-image-generic instead of linux-image-virtual.

This adds a dependency to linux-image-extra-<version>, which brings
a set of kernel modules. Without these modules the generated system
is very limited (for example USB keyboards are not handled).
Echo the truncate command instead of running it in the script.
Detection of the disk (USB stick) on which the OS is running.

The LVM VG the OS is running on is saved in a file /dbstck.conf.
This information is used to retrieve the disk during the first init.