extrausers.bbclass: avoid infinite loop

extrausers.bbclass: avoid infinite loop

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>