useradd.bbclass: Add members to a group

useradd.bbclass: Add members to a group

useradd.bbclass supports adding new users and new groups. But it does not
support adding existing users to existing groups.

There is a need of adding users to some groups (e.g. audio). The class was
extended to call groupmems utility with arguments passed via GROUPMEMS_PARAM.

Signed-off-by: Mikhail Durnev <mikhail_durnev@mentor.com>
Signed-off-by: Saul Wold <sgw@linux.intel.com>