base-files: redirect kill ouptut for ash, telnetd and dropbear

If one of the programmes is not running, then we see the following
output in the logs.

`killall: telnetd: no process killed`

To ensure that the log is clean, redirect the output to /dev/null

Signed-off-by: Florian Eckert <fe@dev.tdt.de>
(cherry picked from commit 40f533b73e)

package/base-files/files/lib/upgrade/stage2

@@ -124,9 +124,9 @@ kill_remaining() { # [ <signal> [ <loop> ] ]
 
 indicate_upgrade
 
-killall -9 telnetd
-killall -9 dropbear
-killall -9 ash
+killall -9 telnetd 2>/dev/null
+killall -9 dropbear 2>/dev/null
+killall -9 ash 2>/dev/null
 
 kill_remaining TERM
 sleep 3