meta: Support opting out of any distro features

meta: Support opting out of any distro features

Add default values to DISTRO_FEATURES using the new
filter_default_features() function.

This change obsoletes the variables DISTRO_FEATURES_BACKFILL and
DISTRO_FEATURES_BACKFILL_CONSIDERED. Instead, all defaults are added via
DISTRO_FEATURES_DEFAULTS and users can opt out of any of these using
DISTRO_FEATURES_OPTED_OUT. Hopefully the variable naming here is easier
for people to understand and remember.

Migration notes:

- If you have previously assigned DISTRO_FEATURES without using
  DISTRO_FEATURES_DEFAULT, you will now get the default features added
  automatically. You will need to review these and add any features you
  do not want to use to DISTRO_FEATURES_OPTED_OUT.

- DISTRO_FEATURES_DEFAULT is now unused, the new variable name is
  slighlty different to ensure that it is not accidentally used if a
  layer hasn't been modified to adapt to the new naming.

- If you previously set DISTRO_FEATURES_BACKFILL_CONSIDERED, use the new
  variable DISTRO_FEATURES_OPTED_OUT instead.

- If you previously modified DISTRO_FEATURES_BACKFILL, don't do that.

Signed-off-by: Paul Barker <paul@pbarker.dev>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>