add CONFIG_LIBXS item

add CONFIG_LIBXS item

Mini-OS contains a stripped down version of libxenstore in lib/xs.c.
Today it is being built always if CONFIG_XENBUS is set and libc
support is enabled.

In order to allow a Mini-OS specific build of libxenstore to be used
instead add a new CONFIG_LIBXS item which per default will have the
same setting as CONFIG_XENBUS. A user wanting to replace lib/xs.c
with libxenstore would just need to set CONFIG_XENBUS=y and
CONFIG_LIBXS=n.

Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Samuel Thibault <samuel.thibault@ens-lyon.org>