ntp: control refclock driver support via PACKAGECONFIG

The default configuration of ntp includes a large number of reference
clock drivers. Provide a PACKAGECONFIG to allow control over whether
or not these refclock drivers are built. Leave enabled by default.

  http://doc.ntp.org/4.2.8/refclock.html



Signed-off-by: Andre McCurdy <armccurdy@gmail.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>
Signed-off-by: Joe MacDonald <joe_macdonald@mentor.com>