package/skeleton-systemd: invert factory logic

Currently, we handle the factory by redirectoring /var with a symlink at
build time, and with some trickery during the filesystem generation,
depending on whether we need to remount the filesystem read-write or
not.

However, this is causing quite some pain with the latest systemd, now that
they have moved their dbus socket to /run instead of /var/run.

As such, trying to play tricks with /var/run as a symlink is difficult,
because at times it is in .usr/share/factory/var/run (during build) and
then it is in /var/run (at runtime). So a relative symlink is not
possible. But an absolute symlink is not possible either, because we are
installing out-of-tree.

Oh the joys of cross-compilation... :-)

We fix all this mess by making /var a real directory from the onset, so
that we can use the runtime-expected layout even during the build.

Then, during filesystem generation, we move /var away to the factory,
and populate it as we used to do. This still requires a post-fs hook to
restore /var after the filesystem generation.

This leaves a situation that, should the filesystem generation fails,
/var will be left in an inconsistent state. But that is not worse than
what we already had anyway.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Maxime Hadjinlian <maxime.hadjinlian@gmail.com>
Cc: Trent Piepho <tpiepho@impinj.com>
Cc: Adam Duskett <aduskett@gmail.com>
Cc: Romain Naour <romain.naour@gmail.com>
Cc: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Cc: Peter Korsgaard <peter@korsgaard.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>