systemd: allow add users to groups in rootfs postprocess
Currently the functionality checks for the "u" and "g" flags to create users and
groups, but not the "m" flag to add users to groups. This change first checks to
be sure that the users and groups are created, creates them if necessary, then
adds the user to the group.
Signed-off-by: Stephano Cetola <stephano.cetola@linux.intel.com> Signed-off-by: Ross Burton <ross.burton@intel.com> Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
