extrausers.bbclass: add a new bbclass

extrausers.bbclass: add a new bbclass

This class is dedicated to image level user/group configuration.
It inherits useradd_base.bbclass.

Users need to inherit this class in their layers or local.conf to
make the setting of EXTRA_USERS_PARAMS effective.

For detailed configuration format of EXTRA_USERS_PARAMS, please
refer to local.conf.sample.extended.

[YOCTO #4074]

(From OE-Core rev: 4b6deb521183b728d9a1c651d4805fe635e6cb50)

Signed-off-by: Chen Qi <Qi.Chen@windriver.com>
Signed-off-by: Saul Wold <sgw@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>