Extend configuration of NTP sources in UCI:
- Add nts option to enable NTS
- Add disabled option to allow inactive sources

Add nts section to UCI with:
- rtccheck option to disable certificate time checks on systems that
  don't have an RTC to avoid the chicken-and-egg problem (it is less
  secure, but still should be better than no NTS at all)
- systemcerts option to disable system certificates
- trustedcerts option to specify path to trusted certificates

Save NTS keys and cookies by default to avoid unnecessary NTS-KE
sessions when restarted or switching back to an already used NTS source.
Also, save the drift to stabilize the clock after chronyd restart.

Signed-off-by: Miroslav Lichvar <mlichvar0@gmail.com>

- fix the init script to read the right config
- rework the init script to allow reusing its code in the hotplug script
- find wan interfaces in the hotplug script instead of using hardcoded
  name and set the online/offline status separately for IPv4/IPv6
- allow NTP access on interfaces that are configured after chronyd start
- add NTP servers obtained from DHCP, options are specified in a new
  dhcp_ntp_server config section
- start chronyd before the network service, include a patch to always
  have IP_FREEBIND defined, which seems to be missing with uclibc

Signed-off-by: Miroslav Lichvar <mlichvar0@gmail.com>

Signed-off-by: Miroslav Lichvar <mlichvar0@gmail.com>