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/scripts/update_submodules

@@ -21,7 +21,7 @@
 
 export LANG=C
 export LC_ALL=C
-export TZ=UTC
+export TZ=UTC0
 
 min_commits=10