util/genbuild_h: Add gitfile support

`genbuild_h.sh` checks whether it is running from a coreboot's own git
worktree to decide whether to use git as the time source. This check
fails when `${top}/.git` is a gitfile, e.g. when coreboot is a
submodule.

Add a proper `git rev-parse` call to check the condition, remove `$top`
which is not used anymore.

Change-Id: I8bb13d607a01f4f28fa8b165769e0a1f702da362
Signed-off-by: Alex Thiessen <alex.thiessen.de+coreboot@gmail.com>
Reviewed-on: https://review.coreboot.org/23256
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>

util/genbuild_h/genbuild_h.sh

@@ -22,13 +22,11 @@ export LANG=C
 export LC_ALL=C
 export TZ=UTC0
 
-top=`dirname $0`/../..
-
 if [ "${BUILD_TIMELESS}" = "1" ]; then
 	GITREV=Timeless
 	TIMESOURCE="fixed"
 	DATE=0
-elif GIT_DIR="${top}/.git" git status > /dev/null 2>&1; then
+elif [ "$(git rev-parse --is-inside-work-tree 2>/dev/null)" = "true" ]; then
 	GITREV=$(LANG= git log -1 --format=format:%h)
 	TIMESOURCE=git
 	DATE=$(git log --pretty=format:%ct -1)