oniro-linux.conf: Rethink how DISTRO_FEATURES/IMAGE_FEATURES are defined

Right now, we do a remove in DISTRO_FEATURES, that, while we don't
neccessarily support them, due to the finality of :remove we force our
end users to not use them either.

By moving DISTRO_FEATURES:remove to DEFAULT_DISTRO_FEATURES:remove, we
allow end users to still use oniro.conf but add in those removed
features.

We also move IMAGE_FEATURES from a distro level var to the image.
IMAGE_FEATURES should generally be put in image recipes. This also gives
end users a way to use Oniro and work around our :remove defaults.

Signed-off-by: Eilís Ní Fhlannagáin <pidge@pidge.org>