base-files: save current time to /etc on poweroff if ntp synced before

even if the RTC fails, we can still keep the time as accurate as possible.

package/base-files/files/etc/hotplug.d/ntp/00-savelatest

@@ -0,0 +1,3 @@
+[ "$ACTION" = "stratum" ] || exit 0
+
+touch /var/state/ntp_last_sync

package/base-files/files/etc/init.d/sysfixtime

@@ -22,6 +22,7 @@ start() {
 }
 
 stop() {
+	[ -e /var/state/ntp_last_sync ] && touch /etc/.last_saved_time
 	hwclock_save
 }