pkg-generic: allow full URLs for <pkg>_EXTRA_DOWNLOADS

The current logic for <pkg>_EXTRA_DOWNLOADS assumes that it is a list
of files, all hosted at <pkg>_SITE. However, just like for
<pkg>_PATCH, it may be useful to specify <pkg>_EXTRA_DOWNLOADS entries
that are hosted on a different site than the package <pkg>_SITE.

This commit implements this, by re-using the same logic as the one
used for <pkg>_PATCH.

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