legal-info: support per-package hooks

Useful to produce extra warnings for packages that have special
licensing-related issues.

Signed-off-by: Luca Ceresoli <luca@lucaceresoli.net>
Acked-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>