ntp: Clean up recipes

Clean up recipes to make them easier to read and to allow ntp-ssl to build
 * Move common portions to ntp.inc
 * Update ntp-ssl to require ntp.inc oppose to ntp_${PV}.bb
 * Change ntp-ssl EXTRA_OECONF to append so it won't try to configure snmp as
it will use local paths can cause a error while configuring

Signed-off-by: Morgan Little <morgan.little@windriver.com>