We need to use 'purge' instead of 'remove' for debian package backend when
removing packages at rootfs time. This is because that 'remove' command for
'dpkg' and 'apt-get' does not remove configuration files.
Signed-off-by: Chen Qi <Qi.Chen@windriver.com> Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>