firmware-upgrade-reboot: add a delay before reboot

Looks like my timing was too good... Upgrade and reboot happened too
fast, so device reported:

system;error;critical router was rebooted without proper shutdown

Let's try something smart... Delay the reboot by the amount of uptime,
and hope all devices - slow and powerful - are happy.

(cherry picked from commit 3f8e835233)

firmware-upgrade-reboot

@@ -28,5 +28,10 @@
   :delay 1s;
 }
 
+:local Uptime [ / system resource get uptime ];
+:if ($Uptime < 1m) do={
+  :delay $Uptime;
+}
+
 $LogPrintExit2 info $0 ("Firmware upgrade successful, rebooting.") false;
 / system reboot;