ntpd: Update scripts to be on par with busybox's ntpd

Update the scripts to work as busybox NTPd, right now the script for getting option 42 is not working.

Make sure we replace the servers instead of appending them like it is done in busybox.

Use OpenWRT interfaces for configuration.

Change-Id: I97e180f5476eed2bd11a2005de61cd0231316c6c