systemd: sed ROOT_HOME only if sysusers PACKAGECONFIG is set

systemd: sed ROOT_HOME only if sysusers PACKAGECONFIG is set

Fixes a bug introducted in ebafe46379 systemd: upgrade to 255.1.

Besides updating systemd, that commit also made other changes. One of them
being when to perform the replacement in order to fix ROOT_HOME.

Previously, that happened on a configure prefunc and on
${S}/sysusers.d/basic.conf.in.
Now it happens in install and on image/usr/lib/sysusers.d/basic.conf.

However, that file is not present if sysusers is not in PACKAGECONFIG,
since that file in that case is not installed hence resulting in:
sed: can't read <redactedpath>/image/usr/lib/sysusers.d/basic.conf: No such file or directory

Previously, in the case of sysusers not being in PACKAGECONFIG, that was a
"silent error" since the replacement was done but the file was not really
used since the file was not installed.

Signed-off-by: Christian Bräuner Sørensen <yocto@bsorensen.net>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>