configure: Rename FILES variable to LINKS

configure: Rename FILES variable to LINKS

The FILES variable is used to accumulate a list of things to symlink
from the source tree into the build tree.  These don't have to be
individual files; symlinking an entire directory of data files is
also fine.  Rename it to something less confusing before we add a few
directories to it.

Improve the comment to clarify what DIRS and LINKS do and why
it's not a good idea to add things to LINKS with wildcarding.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>