ntp: make servers configurable and default to none configured

People can't blindly use pool.ntp.org, especially if they are building
for a product or something that could be used in a product, so at least
try to get people to do the right thing and not use pool.ntp.org by
default.

The previous network interface up script is now installed as
ntpdate-sync in ${bindir} (and symlinked as the network if-up script).
NTPSERVERS needs to be set in /etc/default/ntpdate.default at runtime,
or a different version of this file with a default value can be provided
in a bbappend. (If you intend to use ntpd you will also want to provide
an ntp.conf with the appropriate server(s) set there as well.)

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