base-files: add enabled config option for ntp service

Signed-off-by: Luka Perkov <luka@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@41354 3c298f89-4303-0410-b956-a3cf2f4a3e73
busybox: use ntp enabled config option

Even if enabled option is missing from ntp configuration we are still keeping
default behavior.

Signed-off-by: Luka Perkov <luka@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@41355 3c298f89-4303-0410-b956-a3cf2f4a3e73