Avoid waiting for time synchronization too early (#2594)

In case a system takes a bit longer to boot (e.g. due to SWAP
initialization on first boot, especially on a system with lots of memory
and not very fast strage, e.g. an ODROID-M1 using an SD card) we might
time-out waiting for time synchronization before the time
synchronization service even got started. By ordering the
systemd-time-wait-sync.service after the network is online, the timeout
of this service should be started much later. With that the
systemd-time-wait-sync.service shouldn't timeout any longer.

buildroot-external/rootfs-overlay/etc/systemd/system/systemd-time-wait-sync.service.d/network-online.conf

@@ -1,4 +1,5 @@
 [Unit]
+After=network-online.target
 Wants=network-online.target
 
 [Install]