pkg-generic: fix the logic showing the "Downloading" message

Even though the .stamp_downloaded target is executed for each package
being built, the pkg-generic infrastructure tries to not display the
"Downloading" message when there is in fact nothing to download.

Unfortunately, the logic was incorrect for the patch download: it
forgot the fact that <pkg>_PATCH can contain either file names (in
which case we assume the patch should be downloaded from <pkg>_SITE),
or full URLs. The latter case was not properly handled, as we were
checking if $(DL_DIR)/<full URL> existed, while we should be testing
if $(DL_DIR)/`basename <full URL>` exists.

This patch fixes that, which makes sure the "Downloading" message is
displayed only when necessary.

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>