Reduce timeout for network time synchronization to 15 seconds

The timeout of 90s was introduced before it was ensured that the timesync
systemd unit starts after network is online. Now with that, it makes less sense
to wait that long - if network is unreachable at the point the time
synchronization starts, and the server fails to reply on the first sync, the
polling interval is exponentially increased and the benefit of waiting for more
attempts is doubtful.

Since another synchronization attempt is done after network changes its state,
we should rely on that instead of having the 90 seconds interval as a waiting
period for plugging the network cable. Worst case, there are other mechanisms
that should set the time to a reasonably accurate value, making the NTP sync
less importart for most of the cases.

buildroot-external/rootfs-overlay/usr/lib/systemd/system/systemd-time-wait-sync.service.d/timeout.conf

@@ -1,2 +1,2 @@
 [Service]
-TimeoutStartSec=90s
+TimeoutStartSec=15s