Package downloads: allow restricting to primary site only

Package downloads: allow restricting to primary site only

This patch adds a new config option BR2_PRIMARY_SITE_ONLY that, when set,
restricts package downloads to the specified BR2_PRIMARY_SITE. If the package
is not present on the primary site, the download fails.

This is useful for project developers who want to ensure that the project can
be built even if the upstream tarball locations disappear.

[thomas.petazzoni@free-electrons.com:
 Extend config option help message with more details coming from the
 commit log. Added a dependency on the fact that a primary site has
 been defined. Without any primary site (the default configuration),
 this new option does not make any sense.]

Signed-off-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Acked-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>