native: Fix DISTRO_FEATURES backfill handling in native case

native: Fix DISTRO_FEATURES backfill handling in native case

Currently, features listed in DISTRO_FEATURES_FILTER_NATIVE are not supported
for DISTRO_FEATURES_BACKFILL as the two variables interact badly. We now have
need to add some features to backfill which are in the filter_native case.

This patch fixes the handling by appuing the backfill, then zeroing the variable
once they are applied. This leads to them being correctly filtered.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>