Makefile: use the package infra based external-deps

This commit changes the global 'external-deps' target to use the newly
introduced per-package <pkg>-all-external-deps, instead of relying on
the 'source' target with a custom DL_MODE.

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