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.

This allows us to execute the early config in normal mode and thus,
we can use things like "if" etc.

may have to unmount the partition image to operate cleanly.

This allows make to track the target as dependency. In particular,
it can now be triggered by the target image_content. Hence, it is
no longer neccessary to call it manually

as "fronted" for image_assemble.

Some targets require the image partition to be mounted.
Although, it is easy to detect this (and mount the partition)
by depending on the phony target image_mount_if, make is unable
to calculate the dependencies correctly. The reason is that some
dependencies are located on the partition (which is not mounted
when scanning for dependencies). Hence, make will rebuild all
targets (after it mounted the partition) regardless of the state
of these files.

To solve this issue we introduce ensure_mount and call it on phony
targets invoked by the user. The actual target will be converted and
serve as wrapper which mounts the partition and then invokes the actual
target renamed to <name>__ignore_mount.