useradd.bbclass: avoid do_rootfs error for debian package backend

useradd.bbclass: avoid do_rootfs error for debian package backend

If /etc/login.defs is treated as a configuration file, then we would meet
errors at do_rootfs time telling us that useradd/groupadd cannot execute
correctly.

This is because the dpkg handles config file specially, the login.defs
is temporarily renamed as login.defs.dpkg-new.

How ubuntu deals the user/group adding problem? They do it at postinst of the
package. And, the postinst script of a package would possibly do `chown' of
its files or directories.

The above strategy is not suitable for OE. Because we do chown in do_install
and add user/group in preinst scripts of the packages.

That's why we need this patch so that do_rootfs don't fail.

Signed-off-by: Chen Qi <Qi.Chen@windriver.com>