Makefile: implement the 'source' target using the package infrastructure

Now that all the bits are in place, switch the global 'source' target
to use the package infrastructure logic.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Acked-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Tested-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
[with 'make source' (actually together with the next patch).]
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Tested-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
[tested with a randpackageconfig]

Makefile

@@ -612,7 +612,7 @@ target-post-image: $(TARGETS_ROOTFS) target-finalize
 		$(call MESSAGE,"Executing post-image script $(s)"); \
 		$(EXTRA_ENV) $(s) $(BINARIES_DIR) $(call qstrip,$(BR2_ROOTFS_POST_SCRIPT_ARGS))$(sep))
 
-source: $(PACKAGES_SOURCE) $(HOST_SOURCE)
+source: $(foreach p,$(PACKAGES),$(p)-all-source)
 
 _external-deps: $(foreach p,$(PACKAGES),$(p)-all-external-deps)
 external-deps: