pkg-generic: introduce a <pkg>_ALL_DOWNLOADS variable and factorize code

Now, both the download and source-check code are iterating over
<pkg>_SOURCE, <pkg>_PATCH and <pkg>_EXTRA_DOWNLOADS elements, figuring
out whether they contain full URLs or not. Instead of doing this
repeatdly, this patch introduces an internal <pkg>_ALL_DOWNLOADS
variable, which contains the list of everything that needs to be
downloaded, with URLs already expanded to take into account <pkg>_SITE
if needed.

This allows to simplify quite significantly the .stamp_download and
source-check implementation.

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