meta-systemd: ntp: update for server config improvements

This makes the service use the ntpdate-sync command which reads the
servers set in /etc/default/ntpdate; if nothing is set then no error is
produced on startup.

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>