core/pkg-generic: allow packages to declare target-finalize hooks

core/pkg-generic: allow packages to declare target-finalize hooks

Currently, packages using target finalize hooks must remember that they
need to register their hook in TARGET_FINALIZE_HOOKS
conditionally (otherwise their hook will be triggered even if the
package is disabled).

In order to avoid this potential mistake, this commit introduces a
per-package target-finalize hook variable, in which packages can
register their target-finalize hooks, with the guarantee that they will
only be triggered if the package is enabled.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Reviewed-by: Romain Naour <romain.naour@gmail.com>
[Thomas: rework commit log.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>