stubdom: simplify and fix Makefile

stubdom: simplify and fix Makefile

The stubdom Makefile is setting up links for various libraries. This
is done only once when qemu links are created and each library's links
are updated/created only if the link for the Makefile of the library
isn't already existing. In case a source is added to one library after
doing the first make of stubdom the new source won't be linked by a
new call of make.

Instead of testing the existence of the Makefile link use a make
dependency which will catch changes of the linked Makefile, too.

At the same time don't repeat the same link pattern 7 times but use a
make macro to do the linking.

Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Samuel Thibault <samuel.thibault@ens-lyon.org>
[ wei: move "touch $@" to correct location in do_links ]
Signed-off-by: Wei Liu <wei.liu2@citrix.com>