fs: use a common tarball as base for the other filesystems

fs: use a common tarball as base for the other filesystems

Currently, some filesystems may want to tweak the content of the target
directory, create special device nodes etc... This all means that:

  - the content of the target directory for a specific filesystems may
    depend on whether another filesystem is enabled or not; for example,
    cpio will create a /init script or symlink and a /dev/console node;

  - the filesystems can not be built in parallel, because they may change
    the content of the target directory while another is being assembled.

Furthermore, the same fakeroot script is executed over-and-over-again
for each filesystem, to create the device nodes, the users and their
homes and files, and setting permissions...

We introduce an intermediate tarball, for which we do the full fakeroot
shebang.

That tarball then serves as the base for the other filesystems, with a
very simple fakeroot script that untars the common tarball, and calls
the actual filesystem image generator on that.

Note that we use a very simple tar command to generate the intermediate
tarball, because we are not concerned with reproducibility of the
archive itself (only of the archived files).

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Cc: Arnout Vandecappelle <arnout@mind.be>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>