Makefile: implement a package based source-check target

This commit switches the implementation of the global source-check
target to use a package infrastructure based mechanism, using the
$(1)-all-source-check target added in the previous commit.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Tested-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Reviewed-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>

Makefile

@@ -619,8 +619,7 @@ external-deps:
 	@$(MAKE1) -Bs $(EXTRAMAKEARGS) _external-deps | sort -u
 
 # check if download URLs are outdated
-source-check:
-	$(MAKE1) DL_MODE=SOURCE_CHECK $(EXTRAMAKEARGS) source
+source-check: $(foreach p,$(PACKAGES),$(p)-all-source-check)
 
 legal-info-clean:
 	@rm -fr $(LEGAL_INFO_DIR)