pkg-generic: refactor the "Downloading" message logic

In the .stamp_downloaded hook, the logic to decide whether or not to
display the "Downloading" message is treating the check of
<pkg>_SOURCE as a special case. But in fact, there is no real reason
to do so: the existing loop used for <pkg>_PATCH and
<pkg>_EXTRA_DOWNLOADS could work just as well.

This commit therefore refactors this piece of code, to have a single
loop checking <pkg>_SOURCE, <pkg>_PATCH and <pkg>_EXTRA_DOWNLOADS.

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