classes/image: skip recipe on invalid IMAGE_FEATURES item

If you add an item to EXTRA_IMAGE_FEATURES in your local.conf that is
not supported by image.bbclass itself (such as "tools-sdk" which is
implemented in core-image.bbclass), it can be somewhat annoying to have
the parse fall over if you have a recipe that inherits image.bbclass
only. Change the error from bb.fatal to skip the recipe instead so that
you only see the error when attempting to build the recipe, plus add a
bit of logic to report if the feature is coming in via
EXTRA_IMAGE_FEATURES.

Fixes [YOCTO #5023].

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>