merge_config.sh: create temporary files in /tmp

Creating temporary files in /tmp (or the path pointed by $TMPDIR) allows the
buildroot top directory to be read-only and shareable between multible builds.
This follows what other scripts do, e.g. check-kernel-headers.sh.

Signed-off-by: Henrique Marks <henrique.marks@datacom.ind.br>
Signed-off-by: Carlos Santos <casantos@datacom.ind.br>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>