procd: service_data: Support data within the service itself

Use the same approach than the service_triggers for the service_data.

Signed-off-by: Pierre Lebleu <pme.lebleu@gmail.com>

package/base-files/files/etc/rc.common

@@ -88,6 +88,10 @@ service_triggers() {
 	return 0
 }
 
+service_data() {
+	return 0
+}
+
 service_running() {
 	return 0
 }