Exclude cjdns from config file if it is not specified

src/freedombone-config

   echo "MY_EMAIL_ADDRESS=$MY_EMAIL_ADDRESS" >> $CONFIGURATION_FILE
   echo "LOCAL_NETWORK_STATIC_IP_ADDRESS=$LOCAL_NETWORK_STATIC_IP_ADDRESS" >> $CONFIGURATION_FILE
   echo "ROUTER_IP_ADDRESS=$ROUTER_IP_ADDRESS" >> $CONFIGURATION_FILE
-  echo "ENABLE_CJDNS=$ENABLE_CJDNS" >> $CONFIGURATION_FILE
+  if [ $ENABLE_CJDNS ]; then
+      echo "ENABLE_CJDNS=$ENABLE_CJDNS" >> $CONFIGURATION_FILE
+  fi
   echo "DEBIAN_REPO=$DEBIAN_REPO" >> $CONFIGURATION_FILE
   echo "NAMESERVER1=$NAMESERVER1" >> $CONFIGURATION_FILE
   echo "NAMESERVER2=$NAMESERVER2" >> $CONFIGURATION_FILE