pdist-bootcheck: Keep sending "update in progress"

A synchronous pdist may take some time without anything happening on
the console. At the same time, other messages from systemd might be
emitted, scrolling the original "Update and  reboot required" to a less
prominent place.

Keep sending messages at regular intervals.

pdist/pdist-bootcheck

@@ -34,7 +34,8 @@ elif [ "$need_pdist_and_reboot" ]; then
         rm /var/run/updatecheck.reboot-triggered
         exit 1
     fi
-    echo "Update and reboot required. Please be patient..."
+    echo "Update and reboot required"
+    while true; do sleep 5; echo "System update in progress. Please be patient..."; done &
     netcat $(distmaster) 237
     mxgrub default
     touch /var/run/updatecheck.reboot-triggered