package/pkg-generic: add post-prepare hooks

package/pkg-generic: add post-prepare hooks

Currently, when a package needs to modify files it inherits from its
dependencies, because they contain paths, we can only do that in a
pre- or post-configure hook.

However, whatever is done as part of those hooks, will be accounted
to the package itself, and thus will trigger file-overwrite detection.

So, we need a way to be able to actually modify files before we
start monitoring changes in those files.

We introduce a new set of hooks that an individual package can set,
or that a package infra can set, and that are called right before
we snapshot the state of target, and host (to which staging belongs),

Signed-off-by: Herve Codina <herve.codina@bootlin.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>