net/mwan3: expand ubus to show when last mwan3track check was done

To know how old the ubus output is, add an age parameter which indicats
how old the check informations on the interface are.

Signed-off-by: Florian Eckert <fe@dev.tdt.de>

net/mwan3/files/usr/sbin/mwan3track

@@ -154,6 +154,7 @@ main() {
 		echo "${lost}" > /var/run/mwan3track/$1/LOST
 		echo "${score}" > /var/run/mwan3track/$1/SCORE
 		echo "${turn}" > /var/run/mwan3track/$1/TURN
+		echo "$(date +'%s')" > /var/run/mwan3track/$1/TIME
 
 		host_up_count=0
 		sleep "${sleep_time}" &