The su binary is handled by alternatives but the man page wasn't, so
installing both util-linux-doc and shadow-doc produces errors.
Also use d.expand() to neaten the code.
(From OE-Core rev: 70a161ee88d3d54fec6d59039c181b43f1857dc3)
Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>