package/ntp: sntp time sync script

This patch adds the installation of a startup script if the sntp
utility is selected as an option. The utility is design to do a
one time step/slew adjustment of the system time (similar to the
ntpdate tool http://support.ntp.org/bin/view/Dev/DeprecatingNtpdate

).
One nice benefit over ntpdate is that sntp can run while ntpd is still
running. However, ntpd may still need to be restarted if the time
step was large enough.

The script provides the ability to override the arguments as part of a
/etc/defaults/sntp file.

On a local LAN, the initial large step adjustment took less then
one second to be retrieved and system time updated. If a user already
has a RTC maintaining the time and the system was powered off for
a long period of time, the script assumes a slew adjustment when
+/- 128ms, rather then a time step(jump). This could be further
tuned by a user with the /etc/defaults/sntp configuration file.

One NTP pool server is being set as sntp uses all of the servers
provided when the DNS is resolved as servers to attempt to retrieve
time from before timing out. It looks like currently that is 4 servers
per *pool.ntp.org hostname.

Cc: Oscar Gomez Fuente <oscargomezf@gmail.com>
Signed-off-by: Matthew Weber <matthew.weber@rockwellcollins.com>
Tested-by: Oscar Gomez Fuente <oscargomezf@gmail.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>