added check and automatic creation of /persistent directory during st…

…artup of OpenPlc

This allows to startup in the same way inside and outside of Docker environment.
In docker container it is mount point for volume.
Outside docker, ie. normal linux it will persist all data in /persistent directory

webserver/scripts/start_openplc.sh

@@ -1,4 +1,8 @@
 #!/bin/bash
+if [[ ! -d "/persistent" ]]
+then
+    mkdir /persistent
+fi
 if [[ ! -f "/persistent/dnp3.cfg" ]]
 then
     cp /workdir/webserver/dnp3_default.cfg /persistent/dnp3.cfg