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.

(From OE-Core rev: 6b3bd34bf8c5e511bccfbb64bdd1236e1e7576e3)

Signed-off-by: Mikhail Durnev <mikhail_durnev@mentor.com>
Signed-off-by: Saul Wold <sgw@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>