bsdinstall: Add ntpdate option

When you install a computer for the first time, the date in the CMOS sometimes
not accurate and you need to ntpdate as ntpd will fail a the time difference
is too big.
Add an option in bsdinstall to enable ntpdate that will do that for us.

Reviewed by: allanjude
Differential Revision: https://reviews.freebsd.org/D13149