Allow overriding the VCS exclude list with *_OVERRIDE_SRCDIR_RSYNC_EXCLUSIONS

Allow overriding the VCS exclude list with *_OVERRIDE_SRCDIR_RSYNC_EXCLUSIONS

We have some internal packages which automatically bake a version string
from the git checkout, and we usually combine these with a
*_OVERRIDE_SRCDIR. I would like to let Buildroot *not* skip the .git
directory when picking up sources from the local checkout.  It turns out
that the existing mechanism (*_OVERRIDE_SRCDIR_RSYNC_EXCLUSIONS) only
supports adding to the exclude list because `rsync` simply uses the
first match from the provided filtering rules.

Solve this by using the user-provided values first. If they match, then
`rsync` won't exclude stuff based on the generic VCS exclude patterns.

Signed-off-by: Jan Kundrát <jan.kundrat@cesnet.cz>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>