Ensure that if MACHINE_FEATURES is not set by the machine config that we
don't end up with expansion errors during parsing. Technically since the
introduction of MACHINE_FEATURES_BACKFILL = "rtc" this is unlikely to be
a problem unless "rtc" is also added to
MACHINE_FEATURES_BACKFILL_CONSIDERED, however we should be consistent
with DISTRO_FEATURES which is defaulted in bitbake.conf.
Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
