Add option to configure NTP server(s) used by JetKVM device

[solemnwarning]

Some networks have local NTP servers which may be preferred or mandatory. There is also a DHCP option for NTP servers which the device could use, although support for this in general isn't widespread.

[hertg]

I need this for JetKVM to work without internet access. When I go to network settings in the JetKVM GUI, I see that it correctly received my local NTP-Server configuration through DHCP. But looking at the packets (123/udp), JetKVM doesn't seem to consider it -- it tries to reach out to a bunch of external NTP servers.

Without this feature, HTTPS-mode won't work, because the self-signed certificates are issued for a timespan in the past. Also see #534

[hertg]

Just saw that this may get resolved in #625 and/or #609

[hertg]

My current workaround is to reroute all 123/udp requests coming from JetKVM to my local ntp server, on MikroTik this can be done with:

/ip firewall nat add action=dst-nat chain=dstnat comment="jetkvm ntp redirect to local" dst-port=123 protocol=udp src-address=<JETKVM-IP>/32 to-addresses=<LOCAL-NTP-IP> to-ports=123

[solemnwarning]

I recompiled the firmware image with a modified server list, but obvs. that means I've disabled auto-update for the time being.