tomoyo: Do not generate empty policy files

The Makefile automatically generates the tomoyo policy files, which are
not removed by make clean (because they could have been provided by the
user). Instead of generating the missing files, use /dev/null if a
given file is not provided. Store the default exception_policy in
exception_policy.conf.default.

Acked-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Signed-off-by: Michal Marek <mmarek@suse.cz>