psplash: Avoid mount the psplash tmpfs twice

psplash: Avoid mount the psplash tmpfs twice

The /etc/init.d/psplash.sh will be invoked both in boot and
shutdown/reboot. And the psplash tmpfs will be mounted twice. This
will trigger a bug in umount and let the system hang when
shutdown/reboot. I already made a patch [1] to fix the issue in
umount, but there is no reason for the psplash to do the twice mount.
So also fix it.

[Yocto 13461]

[1] https://lore.kernel.org/util-linux/20190819083022.12289-1-kexin.hao@windriver.com/T/#u

(From OE-Core rev: 5cea0448c5c75b9defc5fc2582e9b0c14e26a4e9)

Signed-off-by: Kevin Hao <kexin.hao@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>