pkg-generic: take into account <pkg>_EXTRA_DOWNLOADS to display "Downloading" message

The .stamp_downloaded target displays the "Downloading" message even
if there is really something to download. However, this logic only
checks for <pkg>_SOURCE and <pkg>_PATCH: it does not check if
something needs to be downloaded in <pkg>_EXTRA_DOWNLOADS.

This commit fixes that, which makes sure that the "Downloading"
message is displayed if one of the items in <pkg>_EXTRA_DOWNLOADS has
not yet been downloaded.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Reviewed-by: Romain Naour <romain.naour@openwide.fr>
Reviewed-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>