useradd.bbclass: add a new base class and code refactor

useradd.bbclass: add a new base class and code refactor

Add a new base class, useradd_base.bbclass, which is mainly a
collection of basic functions for user/group settings.

The useradd_base.bbclass is intended to be inherited by useradd.bbclass
and the extrausers.bbclass to avoid code cuplication.

[YOCTO #4074]

(From OE-Core rev: 2a57bb7e9a7e154578aa7cb9aeebdf398a54ec00)

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>