shadow: split files needed for PAM use into separate package

shadow: split files needed for PAM use into separate package

The rootfs creator automatically removes shadow for read-only
root filesystems, which breaks use of PAM plugins for login and
other process identity management utilities.  Package those programs
and config files separately, so they don't get removed.

Signed-off-by: Matt Madison <matt@madison.systems>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>