archive-release: add support for shipping downloads inside layers

If you specify a layer name in LAYERS_OWN_DOWNLOADS (space separated), the
downloads for this layer will be shipped in ${LAYERDIR}/downloads rather than
in downloads/, and will be archived into ${LAYERNAME}-downloads.tar rather
than ${REPONAME}-downloads.tar (e.g. mel-support-downloads.tar, not
meta-mentor-downloads.tar).

JIRA: SB-3520

Signed-off-by: Christopher Larson <chris_larson@mentor.com>