It's too easy to cause rebuilds because the DISTRO_FEATURES have changed in
meaningless ways (such as re-ordering or duplicate items). Help stop this by
checking if the feature to be back-filled is already present.
(From OE-Core rev: 63c7192119d54b92d908441109ed4e4fff761cba)
Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>