net/mwan3: use uptime for ubus age info

If the date is changed by ntp the age value of mwan3 on ubus could jitter.
Use instead the uptime value from /proc/uptime which will not change during
system run.

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

net/mwan3/files/lib/mwan3/common.sh

@@ -0,0 +1,6 @@
+#!/bin/sh
+
+get_uptime() {
+	local uptime=$(cat /proc/uptime)
+	echo "${uptime%%.*}"
+}