useradd_base.bbclass: Make perform_groupmems handle overlapping usernames
If the name of the last user being part of the group had a name that
was a prefix of the user being added, then perform_groupmems() would
treat it as if the user already existed in the list of users and not
add it.
Reported-by: Peter Henricsson <peter.henricsson@axis.com> Signed-off-by: Peter Kjellerstedt <peter.kjellerstedt@axis.com> Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
