chrony: improve hotplug and init scripts

- 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>

net/chrony/patches/001-freebind_uclibc.patch

@@ -0,0 +1,13 @@
+diff --git a/sysincl.h b/sysincl.h
+index 30e9b48..8fe16c0 100644
+--- a/sysincl.h
++++ b/sysincl.h
+@@ -70,4 +70,8 @@
+ #include <arpa/inet.h>
+ #endif
+ 
++#if defined(LINUX) && !defined(IP_FREEBIND)
++#define IP_FREEBIND 15
++#endif
++
+ #endif /* GOT_SYSINCL_H */