core/pkg-infra: don't enforce site-method for extra downloads

core/pkg-infra: don't enforce site-method for extra downloads

The site method only ever applies to the main download, while extra
downloads are always to be fetched with wget.

However, the site method is prepended to the URL from within the
DOWNLOAD macro (well, a variable evaluated in the DOWNLOAD macro),
which is called for each download of a package, thus effectively
prepending the site method to all downloads, even the extra ones (and
the patches).

We fix that by prepending the site method from within the
generic-package infra, so that it only applies to the main download.

For that, we move the main _SOURCE out of the foreach loop, so that
we can prepend the site-method to it, without impacting the other
downloads.

Reported-by: Luca Ceresoli <luca@lucaceresoli.net>
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Luca Ceresoli <luca@lucaceresoli.net>
Cc: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Cc: Maxime Hadjinlian <maxime.hadjinlian@gmail.com>
Reviewed-by: Luca Ceresoli <luca@lucaceresoli.net>
Tested-by: Luca Ceresoli <luca@lucaceresoli.net>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>