fs/cpio: use a post-gen hook rather than a post-target rule

fs/cpio: use a post-gen hook rather than a post-target rule

We already have a mechanism for running stuff after the filesystem is
generated, and that's called post-gen hooks.

Use those hooks.

Note: for cpio (and unlike ext2 previously), the dependency chain was
correct, in that the post-target rule correctly depended on the image
rule. Nonetheless, we still want to fix it for consistency.

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>