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

configure

@@ -3479,7 +3479,9 @@ for bios_file in $source_path/pc-bios/*.bin $source_path/pc-bios/*.rom $source_p
 done
 mkdir -p $DIRS
 for f in $FILES ; do
-    test -e $f || symlink $source_path/$f $f
+    if [ -e "$source_path/$f" ] && ! [ -e "$f" ]; then
+        symlink "$source_path/$f" "$f"
+    fi
 done
 
 # temporary config to build submodules