genbuild_h.sh: allow coreboot to be a git submodule

When coreboot is pulled in as a submodule, the .git "folder" is a file,
not a folder.  Use the '-e' test instead of '-d' to allow for that.

Change-Id: I0dd8866b0016f7ba099cdaf4d7db442ff22612b5
Signed-off-by: Ben Gardner <gardner.ben@gmail.com>
Reviewed-on: https://review.coreboot.org/12819
Tested-by: build bot (Jenkins)
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-by: Patrick Georgi <pgeorgi@google.com>
diff --git a/util/genbuild_h/genbuild_h.sh b/util/genbuild_h/genbuild_h.sh
index 4c8f602..7f0c768 100755
--- a/util/genbuild_h/genbuild_h.sh
+++ b/util/genbuild_h/genbuild_h.sh
@@ -24,7 +24,7 @@
 
 top=`dirname $0`/../..
 
-if [ -d "${top}/.git" -a -x "$(command -v git)" ]; then
+if [ -e "${top}/.git" -a -x "$(command -v git)" ]; then
 	GITREV=$(LANG= git log -1 --format=format:%h)
 	TIMESOURCE=git
 	DATE=$(git log --pretty=format:%ct -1)