Revert "Use local timezone for builds"

This reverts commit 1484476552.
It is unclear, why this behaviour had been desired and nowadays with
containers the timezone in the container is UTC anyways no matter where
in the world a build is started.

In any case, the output of the build should be reproducible and the
timezone should not produce a different result, so setting it to UTC
makes sense.

Signed-off-by: Andreas Gnau <andreas.gnau@iopsys.eu>

Makefile

@@ -5,7 +5,8 @@
 TOPDIR:=${CURDIR}
 LC_ALL:=C
 LANG:=C
-export TOPDIR LC_ALL LANG
+TZ:=UTC
+export TOPDIR LC_ALL LANG TZ
 
 empty:=
 space:= $(empty) $(empty)