configure: Don't create symlinks to nonexistent targets

When we create the symlinks to source tree files, don't create them
if the file is not actually present in the source tree; this will
happen if the file is in a git submodule that wasn't checked out.

This also avoids the odd effect where an in-source-tree configure
will end up creating the missing file as a symlink to itself.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
