fs/common: get rid of post-target rules

fs/common: get rid of post-target rules

The only users of post-target rules were ext2, cpio and initramfs.

Of those, ext2 and cpio were changed to use post-gen hooks, while
initramfs was not even using the generic rootfs infra and was fixed
to no longer reference post-target rules.

Besides, the comment in the infra was really misleading: it referenced
initramfs implying it was the sole user of that feature, even though
initramfs was not using the fs infra.

Furthermore, using post-target rules was inherently broken for top-level
parallel builds, because filesystems had to ensure the ordering by
themselves. Of the two real users of post-target rules (cpio and ext2),
one did enforce rules ordering (apparently correctly), while the other
forgot to do so.

We can get rid of post-target rules altogether, now.

Add a legacy check, to catch out-of-tree (e.g. br2-external) users of
post-target rules, and instruct them to switch to post-gen hooks instead.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Arnout Vandecappelle <arnout@mind.be>
Reviewed-by: Romain Naour <romain.naour@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>