psplash: consider the situation of psplash not exist for systemd

In current psplash framework, the psplash might not exist at all.
For example, in case DSITRO is set to nodistro, the psplash does
not exist.

In our psplash recipe, we have:
SPLASH_IMAGES = "file://psplash-poky-img.h;outsuffix=default"
This variable is parsed to if psplash-poky-img.h exists, a package
named psplash-default is created and is added to RDEPENDS:${PN}.

We can see that the psplash-poky-img.h resides in meta-poky,
and in psplash_git.bbappend file in meta-poky, we have:
FILESEXTRAPATHS:prepend:poky := "${THISDIR}/files:"
So this file is only available in case poky distro is used.

To fix this issue, add condition check in the corresponding systemd
services.

Signed-off-by: Chen Qi <Qi.Chen@windriver.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
(cherry picked from commit 7a62ff9e)
Signed-off-by: Steve Sakoman <steve@sakoman.com>