syncrepo.sh: Use rsync-repos from a local checkout

Allow using the rsync-repos script from a local checkout of the releng
scripts repository. While at it, add the script as a submodule for
convenience.

.gitmodules

@@ -0,0 +1,3 @@
+[submodule "ostree-releng-scripts"]
+	path = ostree-releng-scripts
+	url = https://github.com/ostreedev/ostree-releng-scripts

syncrepo.sh

@@ -10,7 +10,8 @@ set -e
 LOCAL="$(dirname "$0")/.flatpak-repo/"
 REMOTE='[email protected]:/srv/http/flatpak.perezdecastro.org/revolt/'
 
-RSYNC_REPOS=$(type -P ostree-rsync-repos rsync-repos | head -1)
+RSYNC_REPOS=$(type -P ostree-rsync-repos rsync-repos \
+	"$(dirname "$0")/ostree-releng-scripts/rsync-repos" | head -1)
 if [[ -z ${RSYNC_REPOS} ]] ; then
 	echo "$0: The ostree-rsync-repos (or rsync-repos) tool is not installed."
 	echo 'Please install from https://github.com/ostreedev/ostree-releng-scripts'