rootfs-postcommands: amend systemd_create_users add user to group check

Currently when adding a user to a group ('m' type), the conditional
check to only create a user/group if it does not exist always resolves
to true. This causes a build exit failure if the user and/or group
defined in the sysusers configuration file were already created prior to
the execution of systemd_create_users().

This logic has been updated to instead fail silently (consistent with
'u' and 'g' type). Additionally, if a user doesn't exist it will be
created without the default group.

Signed-off-by: Tean Cunningham <tean.cunningham@digi.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>