systemd: Fix the problem of an empty journal on boot

systemd: Fix the problem of an empty journal on boot

systemd by default tries to write the journal to /var/log/journal.
But base-files has a symlink /var/log -> /var/volatile/log. And
/var/volatile is a tmpfs mount in /etc/fstab.

If the journal service started before /var/volatile was mounted (which
was the typical scenario) then the journal would appear empty since
the old location was mounted over.

This change fixes the problem by ensuring that the journal doesn't start
until after the mount happens.

[Yocto #7388]

(From OE-Core rev: 17e8595af2041cfd63adf73b344f7ccad3db7e01)

Signed-off-by: Randy Witt <randy.e.witt@linux.intel.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>