dmsquash-live: do not abort, if user pressed ESC on checkisomd5

If the user pressed ESC while checkisomd5 runs the media check, it will
exit with "2". Previously that would mean, that the media check was not
successful.

modules.d/90dmsquash-live/checkisomd5@.service

@@ -11,3 +11,4 @@ StandardInput=tty-force
 StandardOutput=inherit
 StandardError=inherit
 TimeoutSec=0
+SuccessExitStatus=2

modules.d/90dmsquash-live/dmsquash-live-root.sh

@@ -42,7 +42,7 @@ if [ -n "$check" ]; then
     else
         checkisomd5 --verbose $livedev
     fi
-    if [ $? -ne 0 ]; then
+    if [ $? -eq 1 ]; then
         die "CD check failed!"
         exit 1
     fi