ntp: add ntpdate option

Closes #2935

Add ntpdate option and make ntpd optional.

Based on incomplete patch by Frederik Pasch <fpasch@googlemail.com>

Also enable crypto when openssl is enabled.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>