How backfilling works.

DISTRO_FEATURES_BACKFILL = "pulseaudio sysvinit gobject-introspection-data ldconfig"
MACHINE_FEATURES_BACKFILL = "rtc qemu-usermode"

def features_backfill(var,d):
    # This construct allows the addition of new features to variable specified
    # as var
    # Example for var = "DISTRO_FEATURES"
    # This construct allows the addition of new features to DISTRO_FEATURES
    # that if not present would disable existing functionality, without
    # disturbing distributions that have already set DISTRO_FEATURES.
    # Distributions wanting to elide a value in DISTRO_FEATURES_BACKFILL should
    # add the feature to DISTRO_FEATURES_BACKFILL_CONSIDERED
    features = (d.getVar(var) or "").split()
    backfill = (d.getVar(var+"_BACKFILL") or "").split()
    considered = (d.getVar(var+"_BACKFILL_CONSIDERED") or "").split()

    addfeatures = []
    for feature in backfill:
        if feature not in features and feature not in considered:
            addfeatures.append(feature)

    if addfeatures:
        d.appendVar(var, " " + " ".join(addfeatures))