ASoC: SOF: Intel: Broadwell: clarify mutual exclusion with legacy driver

Some distros select all options blindly, which leads to confusion and
bug reports. SOF does not fully support Broadwell due to firmware
dependencies, the machine drivers can only support one option, and
UCM/topology files are still being propagated to downstream distros,
so make SOF on Broadwell an opt-in option that first require distros
to opt-out of existing defaults.

Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=204237
Fixes: f35bf70f

 ('ASoC: Intel: Make sure BDW based machine drivers build for SOF')
Signed-off-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Link: https://lore.kernel.org/r/20191101173045.27099-3-pierre-louis.bossart@linux.intel.com
Signed-off-by: Mark Brown <broonie@kernel.org>