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>
(cherry picked from commit a27078d32d1e23963ff50cd0ab014d033b287af7)
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/pkg-generic.mk

@@ -184,7 +184,7 @@ $(BUILD_DIR)/%/.stamp_rsynced:
 	@mkdir -p $(@D)
 	$(foreach hook,$($(PKG)_PRE_RSYNC_HOOKS),$(call $(hook))$(sep))
 	@test -d $(SRCDIR) || (echo "ERROR: $(SRCDIR) does not exist" ; exit 1)
-	rsync -au --chmod=u=rwX,go=rX $(RSYNC_VCS_EXCLUSIONS) $($(PKG)_OVERRIDE_SRCDIR_RSYNC_EXCLUSIONS) $(call qstrip,$(SRCDIR))/ $(@D)
+	rsync -au --chmod=u=rwX,go=rX $($(PKG)_OVERRIDE_SRCDIR_RSYNC_EXCLUSIONS) $(RSYNC_VCS_EXCLUSIONS) $(call qstrip,$(SRCDIR))/ $(@D)
 	$(foreach hook,$($(PKG)_POST_RSYNC_HOOKS),$(call $(hook))$(sep))
 	@$(call step_end,rsync)
 	$(Q)touch $@