Allow to put a disk.sh script as part of the image description
which is called for the disk image types `vmx` and `oem`
only and runs after the synchronisation of the root tree into the
disk image loop file. At call time of the script the device name
of the currently mapped root device is passed as a parameter.
The chroot environment for the call is the mounted disk itself
which makes this different from the config.sh/images.sh caller
environment. This Fixes#1464
