shadow: use update-alternatives to handle su.1 and nologin.8

shadow: use update-alternatives to handle su.1 and nologin.8

This patch fixes the following error at do_rootfs:

  update-alternatives: Error: not linking
  /PATH/TO/rootfs/usr/share/man/man1/su.1
  to /usr/share/man/man1/su.1.util-linux since
  /PATH/TO/rootfs/usr/share/man/man1/su.1
  exists and is not a link

  update-alternatives: Error: not linking
  /PATH/TO/rootfs/usr/share/man/man8/nologin.8
  to /usr/share/man/man8/nologin.8.util-linux since
  /PATH/TO/rootfs/usr/share/man/man8/nologin.8
  exists and is not a link

The problem can be reproduced by adding the following lines to local.conf
and then building an image:

  EXTRA_IMAGE_FEATURES:append = " doc-pkgs"
  IMAGE_INSTALL:append = " shadow util-linux"

su.1 and nologin.8 are handled by update-alternatives in util-linux recipe, so
do it in shadow recipe too.

Signed-off-by: Sid-Ali Teir <sidali.teir@cysec.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit 198c4582c6391ac87c49e09882189235c44b60e9)
Signed-off-by: Steve Sakoman <steve@sakoman.com>