conf/layer.conf: provide default PREFERRED_PROVIDER_android-tools-conf
Fix the following warning by providing backwards-compatible default for
android-tools-conf preference:
NOTE: Multiple providers are available for runtime android-tools-conf (android-tools-conf, android-tools-conf-configfs)
Consider defining a PREFERRED_RPROVIDER entry to match android-tools-conf
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Signed-off-by: Khem Raj <raj.khem@gmail.com>