boot/barebox, package/linux-headers: carry site-method archive format version when...

boot/barebox, package/linux-headers: carry site-method archive format version when overriding _SOURCE

Commit 5b95a5dc2 (support/download: change format of archives generated
from git) changed the way the archives generated from git repositories
are named, adding a "format-version" identifier right between the
package version and the file extension.

Commit c043ecb20 (support/download: change format of archives generated
from svn) did so for archives generated from a subversion checkout.

However, for a few packages, we manually force the _SOURCE variable,
because we want to share the archive with another package, to avoid
downloading and storing those archives twice. This is the case for:

  - linux-headers and linux
  - barebox-aux and barebox

When the generated tarballs were renamed with the aforementioned
commits, those packages were not updated accordingly.

Fix that by manually propagating the per-site-method format-version.

Reported-by: "Stephane Viau (OSS)" <stephane.viau@oss.nxp.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
Cc: "Stephane Viau (OSS)" <stephane.viau@oss.nxp.com>
Cc: Arnout Vandecappelle <arnout@mind.be>
Cc: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Cc: Peter Korsgaard <peter@korsgaard.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>