buildroot

coreutils: add optionals, use single binary

Add proper support for optional libraries: acl, attr, gmp & libcap.

Also build the single binary as pointed out by Pádraig Brady for some
neat space savings.
Use the shebang (default) method since it allows us to move binaries
into other directories to suit our needs whereas for symlink that
wouldn't be so easy.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>