Avoid infinite loop if the last record in EXTRA_USRES_PARAMS doesn't
end with a semicolon.

It's possible the the users will write configurations like below.

INHERIT += "extrausers"
EXTRA_USERS_PARAMS = "useradd tester; useradd developer"

In such situation, the do_rootfs task will enter an infinite loop.
An infinite loop is never acceptable.

This patch fixes the above problem.

Signed-off-by: Chen Qi <Qi.Chen@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

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]

Signed-off-by: Chen Qi <Qi.Chen@windriver.com>
Signed-off-by: Saul Wold <sgw@linux.intel.com>