Check wifi interface exists

src/freedombone-utils-wifi

         exit 4
     fi
 
+    if [[ $(config_param_exists "WIFI_INTERFACE") == "0" ]]; then
+        exit 5
+    fi
+    read_config_param "WIFI_INTERFACE"
+
     if [ ! -f /etc/network/interfaces_original ]; then
         if ! grep -q "# wifi enabled" /etc/network/interfaces; then
             cp /etc/network/interfaces /etc/network/interfaces_original