fix buildversion.sh

Recent git versions place the submodule git repos into the
.git/modules directory of the toplevel repo.  In that case
.git of the seabios tree isn't a directory, but a regular
file, saying where the gitdir is.

Extent the git check to also allow .git being a regular file,
so buildversion.sh works correctly when called within the qemu
submodule.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
(cherry picked from commit 0b3607f89ac24f84f8b22ffaadb4f0fd111bda24)

tools/buildversion.sh

diff --git a/tools/buildversion.sh b/tools/buildversion.sh
index c8c1725..e5ce96c 100755
--- a/tools/buildversion.sh
+++ b/tools/buildversion.sh
@@ -4,7 +4,7 @@
 VAR16MODE="$2"
 
 # Extract version info
-if [ -d .git ]; then
+if [ -d .git -o -f .git ]; then
     VERSION="`git describe --tags --long --dirty`"
 elif [ -f .version ]; then
     VERSION="`cat .version`"