busybox: don't install NTP scripts if NTP isn't configured

If you're using Chrony or NTPD you don't want the busybox NTP server
as well.  Make it's installation truly conditional.

Signed-off-by: Philip Prindeville <philipp@redfish-solutions.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name> [simplify]