buildroot

packages: use the <PKG>_TARGET_FINALIZE_HOOKS

Register package-specific target-finalize hooks with the
newly-introduced <PKG>_TARGET_FINALIZE_HOOKS.

This incidentally fixes luarocks, which was registering target-finalize
hooks even when it was not enabled.

To be noted, the skeleton package is not converted, because it is not
optional, we always have it; so its hooks would always be registered
anyway. Besides, the followup patches would render this conversion moot
anyway, since those hooks would be spread across the various skeleton
packages.

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