systemd waiting long to shutdown SWUpdate

Under certain situations, systemd is waiting for a long time that
SWUpdate stops. SWUpdate has in fact its own process control management
to monitor its own processes and SIGTERM cannot be sent to the children
by systemd. Add KillMode=mixed option to let SWUpdate to make its job:
systemd will send SIGTERM just to SWUPdate's main process.

Signed-off-by: Stefano Babic <sbabic@denx.de>
Signed-off-by: Johannes Schrimpf <johannes.schrimpf@blueye.no>

recipes-support/swupdate/swupdate/swupdate.service

@@ -7,6 +7,7 @@ Documentation=https://sbabic.github.io/swupdate
 ExecStartPre=-/usr/bin/swupdate-env
 EnvironmentFile=-/tmp/swupdate.env
 ExecStart=/usr/bin/swupdate -v -w "-r /www" $SWUPDATE_EXTRA_ARGS
+KillMode=mixed
 
 [Install]
 WantedBy=multi-user.target