pkg-generic: implement <pkg>-all-{source, legal-info, external-deps} targets

In order to implement the global 'make source', 'make legal-info' and
'make external-deps' using the package infrastructure logic, this
commit introduces a set of per-package targets that allow to
recursively run the source/legal-info/external-deps actions on the
dependencies of a given package.

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