chrony: improve configuration

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>

net/chrony/files/chrony.config

@@ -5,6 +5,7 @@ config pool
 
 config dhcp_ntp_server
 	option iburst 'yes'
+	option disabled 'no'
 
 config allow
 	option interface 'lan'
@@ -12,3 +13,7 @@ config allow
 config makestep
 	option threshold '1.0'
 	option limit '3'
+
+config nts
+	option rtccheck 'yes'
+	option systemcerts 'yes'