busybox: sysntpd - Support for NTP servers received via DHCP(v6)

The busybox ntpd utility currently uses ntp servers specified in uci.
This patch allows the ntpd utility to use NTP servers received via DHCP(v6)
Following uci parameters have been added:
    use_dhcp : enables NTP server config via DHCP(v6)
    dhcp_interface : use NTP servers received only on the specified DHCP(v6) interfaces; if empty all interfaces are considered

Signed-off-by: Hans Dedecker <dedeckeh@gmail.com>