Expose the CONFIG_TCG selector to let minikconf.py uses it.

When building with --disable-tcg build, this helps to deselect
devices that are TCG-dependent.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>