tools/shim: Apply more duct tape to the linkfarm logic

tools/shim: Apply more duct tape to the linkfarm logic

Sander reported a build failure which manifests as `make; make install`
failing with:

  <snip>/cross-install -m0644 -p xen-dir/xen-shim //usr/local/lib/xen/boot/xen-shim
  install: cannot stat 'xen-dir/xen-shim': No such file or directory
  make[4]: *** [Makefile:52: install] Error 1
  make[4]: Leaving directory '/usr/src/new/xen-unstable/tools/firmware'

It has subsequently been seen intermittently by OSSTest.  This was caused by
c/s 32b1d628 triggering a preexisting linkfarm bug for partial rebuilds.

Between the first `make` and the subsequent `make install`, the linkfarm logic
observes new final build products and regenerates the linkfarm.  This includes
a distclean, which throws away everything from the first `make`.

As the xen-shim rule use a symlink, the link itself remains still up-to-date
but is broken due to the distclean, which causes install to fail.

Update the linkfarm logic to not regenerate itself when build artefacts
appear.  This isn't a comprehensive fix but is the best which can be done
easily.  Any further effort would be better spent making out-of-tree builds
work for Xen.

Reported-by: Sander Eikelenboom <linux@eikelenboom.it>
Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>