util: Make TZ environment variable POSIX compatible

`TZ='UTC'` is not a portable setting for the TZ environment variable.
POSIX says you’re supposed to use something like `TZ='UTC0'` instead.
Although `TZ='UTC'` works when GLIBC is used, this is not necessarily
true on other POSIX platforms. [1][2]

[1] http://lists.alioth.debian.org/pipermail/reproducible-builds/Week-of-Mon-20170918/009289.html
[2] http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap08.html#tag_08_03

Change-Id: I1dca0b84de0ec0af3a103e2cbbf731512eb59497
Signed-off-by: Paul Menzel <pmenzel@molgen.mpg.de>
Reviewed-on: https://review.coreboot.org/21721
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Jonathan Neuschäfer <j.neuschaefer@gmx.net>
Reviewed-by: Martin Roth <martinroth@google.com>

util/genbuild_h/genbuild_h.sh

@@ -20,7 +20,7 @@ TIMESOURCE=""
 
 export LANG=C
 export LC_ALL=C
-export TZ=UTC
+export TZ=UTC0
 
 top=`dirname $0`/../..
 
@@ -35,7 +35,7 @@ elif [ -e "${top}/.git" -a -x "$(command -v git)" ]; then
 else
 	GITREV=Unknown
 	TIMESOURCE="date"
-	DATE=$(LANG= LC_ALL=C TZ=UTC date +%s)
+	DATE=$(LANG= LC_ALL=C TZ=UTC0 date +%s)
 fi
 
 our_date() {