ntp: create /tmp/sync_status when ntp is synchronized + webui

[nicolasroche898]

Add a file /tmp/sync_status that signals that ntp time is synchronized.

Scripts that depend on synchronized time, for example writing files with correct timestamps, can wait for existence of the file using:

#!/bin/sh

# wait for time synchronization:

while [ ! -e "/tmp/sync_status" ]; do
    sleep 1;
done

echo "time synchronized! proceeding..."

Added also current sync status in config-time web ui: