ntp: enable/disable sntp support depending on BR2_PACKAGE_NTP_SNTP

ntp: enable/disable sntp support depending on BR2_PACKAGE_NTP_SNTP

We already have an option for selecting sntp support in ntp that can be
chosen from the menuconfig, and ntp's configure script has a --with-sntp
option (with its --without counterpart) which can be used for disabling
sntp support in ntp. However, we are not using it. This patch will make
use of it.

Signed-off-by: Vicente Olivert Riera <Vincent.Riera@imgtec.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>