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]