Changes to show the best way to use this variable in relation
to the EXTRA_IMAGE_FEATURES variable.
(From yocto-docs rev: 3afa91f8fdecae18320364d9332639e725ecef5a)
Signed-off-by: Scott Rifenbark <scott.m.rifenbark@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>