genbuild_h: timestamps should not depend on LANG or LC_ALL

Fixes reproducible builds.

Change-Id: I3b0a21f93daee09605c0c2a05c1739e04f44527f
Signed-off-by: Alexander Couzens <lynxis@fe80.eu>
Reviewed-on: http://review.coreboot.org/10447
Tested-by: build bot (Jenkins)
Reviewed-by: Peter Stuge <peter@stuge.se>

util/genbuild_h/genbuild_h.sh

@@ -27,7 +27,7 @@ if [ -d "${top}/.git" ] && [ -f "$(command -v git)" ]; then
 	DATE=$(git log --pretty=format:%ct -1)
 else
 	GITREV=Unknown
-	TIMESOURCE="LANG= TZ=UTC date"
+	TIMESOURCE="LANG=C LC_ALL=C TZ=UTC date"
 	DATE=$(date +%s)
 fi