Omit /var/log/journal from rootfs image

The persistent journal is stored in a subdirectory of `/var/log/journal`
named for the current machine ID.  Since `/etc/machine-id` is not
writable, the machine ID changes with every boot.  This effectively
makes the journal for previous boots inaccessible, so there's really not
much point in keeping them around.

yellow/squashfs.exclude

@@ -54,3 +54,4 @@ var/cache/edb
 var/db/Makefile
 var/db/pkg
 var/lib/portage
+var/log/journal