which, additionally, serves as an example on how to start from the current
daily isos.

Without this casper would uninstall some packages at the end of the
installation we still need when installing packages from the offline repository

This should fix that the progress bar shows 100% for most of the installation
process.

Reference: https://help.ubuntu.com/community/LiveCDCustomizationFromScratch

disabled singular
updated vanilla kernel urls

such that linuxparty.py can access it.

This is required because repo_packages has to access this file from
inside the remaster container (contrib is copied over while config is not)

The kernel xz implementation can only handle crc32 or none integrity
check but the xz default is crc64 and the kernel will panic.

xz is faster (in particular because it uses multiple cpu cores)
and the resulting initrd has the same size (up to 0.5MB)

findmnt always returns the absolute path to a mount point, so
we have to compare the result with an absolute path.
But $(IMAGE_DIR) may be a relative path; thus, convert it to an
absolute path first.

This fixes the "No space left on device" error that occured when
updating large files (e.g. a squashfs) on the image partition file.

The offical singular-version is either uses old packages
or installs automatically package sources. The Debian-Version
seems to be reasonable sane.