package/dracut: workaround breakage on non-merged-usr hosts

dracut is not really ready to be installed with a non-/ prefix, and it
has a lot of hard-coded assumptions that it is going to run on the host
for which it is goign to generate an initramfs; for example, it
hard-codes calls to /lib/dracut/some-file in some of its modules. It
also uses the host system layout to decide whether it needs a
merged-usr or not.

Furthermore, dracut populates the temporary directory which content will
be used to generate the cpio, with a bunch of files, even before calling
any of the dracut modules.

The name for that temporary directory is not predictable (looks like the
output of 'mktemp -d dracut.XXXXXX', with names like dracut.1Vfn9F seen
while debugging).

As a consequence, we can't prepare the temporary directory with the
proper symlinks beforehand.

So, we provide a very-early module of our own, that will (hopefully) run
before any other module, to fixup the messed-up layout prepared by
dracut. This module moves the content of /lib, /bin, and /sbin, out and
into their counterparts in /usr, and creates the usual symlinks.

When we do not require a merged-usr, then we have nothing to do, so the
module checks for /lib being a symlink, as the hint that we want a
merged-usr or not.

Note: currently, we've seen nothing that dracut installed in /bin or
/sbin, but for trying to be future-proof, we also handle them; this
causes a spurious warning:
    mv: cannot stat '..../build/buildroot-fs/cpio/tmp/dracut.YQnzNP/initramfs/bin/*': No such file or directory

Since there are already quite a bunch of similar failures in the
official modules bundled in dracut, an extra such issue or two should
not be too scary...

Fixes:
    https://gitlab.com/buildroot.org/buildroot/-/jobs/3282261241
    https://gitlab.com/buildroot.org/buildroot/-/jobs/3282261239
    https://gitlab.com/buildroot.org/buildroot/-/jobs/3282261236



Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
Cc: Thierry Bultel <thierry.bultel@linatsea.fr>
Cc: Adam Duskett <aduskett@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>