Revert the usage of file-resource back to script

The file-resource doesn't work on first install because then the package
syslinux is only staged for installation but the file-resource wants to read
the file during parsing.
Create a working boot-menu.

This gives three (working) choices:
 - Restore a whole disk and ask only for source image and target disk.
 - Restore a partition and ask only for source image and target partition.
 - Run full Clonezilla with write-access on the clonezilla-server to write
   images.