scripts: Speed up uncrustify by running 4 jobs in parallel

There are some large files, and in most setups (including CI runners) we
have multiple cores available. Use xargs to run multiple parallel
uncrustify jobs rather than one large one. Just hardcode 4 jobs and 4
files at the same time for now.

scripts/uncrustify.sh

@@ -12,8 +12,11 @@ case "$1" in
         ;;
 esac
 
+ARGS=4
+JOBS=4
+
 pushd "$SRCROOT"
-uncrustify -c "$CFG" $OPTS `git ls-tree --name-only -r HEAD | grep -E '.*\.[ch]$' | grep -v nbis | grep -v fpi-byte | grep -v build/`
+git ls-tree --name-only -r HEAD | grep -E '.*\.[ch]$' | grep -v nbis | grep -v fpi-byte | grep -v build/ | xargs -n$ARGS -P $JOBS uncrustify -c "$CFG" $OPTS
 RES=$?
 popd
-exit $RES
+exit $RES