systemd: Store journal log data only in memory

From `man journald.conf`:

> Storage=
>     Controls where to store journal data. One of "volatile",
>     "persistent", "auto" and "none". If "volatile", journal log data
>      will be stored only in memory, i.e. below the /run/log/journal
>      hierarchy (which is created if needed). If "persistent", data will
>      be stored preferably on disk, i.e. below the /var/log/journal
>      hierarchy (which is created if needed), with a fallback to
>      /run/log/journal (which is created if needed), during early boot
>      and if the disk is not writable.  "auto" is similar to
>      "persistent" but the directory /var/log/journal is not created if
>      needed, so that its existence controls where log data goes.
>      "none" turns off all storage, all log data received will be
>      dropped. Forwarding to other targets, such as the console, the
>      kernel log buffer, or a syslog socket will still work however.
>      Defaults to "auto".

systemd.be0

@@ -101,6 +101,9 @@ mee_install_post() {
 
     # Remove directory to disable persistent storage of logs
     rm -rv ${D}/var/log/journal
+
+    # Explicitly configure volatile storage of logs
+    start_cmd sed -i 's/^#Storage=auto/Storage=volatile/' ${D}/etc/systemd/journald.conf
 }
 
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