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/board_status/to-wiki/towiki.sh

@@ -19,7 +19,7 @@ while read line; do
 		commit=`echo $i | cut -d/ -f3`
 		datetime_path=`echo $i | cut -d/ -f4`
 		datetime=`echo $datetime_path | tr _ :`
-		datetime_human=`LC_ALL=C TZ=UTC date --date="$datetime"`
+		datetime_human=`LC_ALL=C TZ=UTC0 date --date="$datetime"`
 		upstream=`grep "^Upstream revision:" $vendor_board/$commit/$datetime_path/revision.txt |cut -d: -f2-`
 		upstream=`git log -1 --format=%H $upstream`
 		if ! echo "$have"| grep  "^$vendor_board:" > /dev/null; then