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>

package/pkg-generic.mk

@@ -587,6 +587,12 @@ $(1)-graph-depends: graph-depends-requirements
 			|tee $$(GRAPHS_DIR)/$$(@).dot \
 			|dot $$(BR2_GRAPH_DOT_OPTS) -T$$(BR_GRAPH_OUT) -o $$(GRAPHS_DIR)/$$(@).$$(BR_GRAPH_OUT)
 
+$(1)-all-source:       $$(foreach p,$$($(2)_FINAL_DEPENDENCIES),$$(p)-all-source) $(1)-source
+
+$(1)-all-external-deps:        $$(foreach p,$$($(2)_FINAL_DEPENDENCIES),$$(p)-all-external-deps) $(1)-external-deps
+
+$(1)-all-legal-info:   $$(foreach p,$$($(2)_FINAL_DEPENDENCIES),$$(p)-all-legal-info) $(1)-legal-info
+
 $(1)-dirclean:		$$($(2)_TARGET_DIRCLEAN)
 
 $(1)-clean-for-reinstall: