genbuild_h: actually make date(1) based timestamp locale independent

This fixes the botched fix in commit d9bc2fadc4

Change-Id: I0c4445af2851bc80fabb631864321a56123ce7b0
Signed-off-by: Patrick Georgi <patrick@georgi-clan.de>
Reviewed-on: http://review.coreboot.org/11146
Tested-by: build bot (Jenkins)
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>

util/genbuild_h/genbuild_h.sh

@@ -32,8 +32,8 @@ if [ -d "${top}/.git" ] && [ -f "$(command -v git)" ]; then
 	DATE=$(git log --pretty=format:%ct -1)
 else
 	GITREV=Unknown
-	TIMESOURCE="LANG=C LC_ALL=C TZ=UTC date"
-	DATE=$(date +%s)
+	TIMESOURCE="date"
+	DATE=$(LANG= LC_ALL=C TZ=UTC date +%s)
 fi
 
 our_date() {