classes/image: show an error on invalid IMAGE_FEATURES
If the user specifies an invalid feature in IMAGE_FEATURES, show an
error during parsing. Valid IMAGE_FEATURES are drawn from
PACKAGE_GROUP_ definitions, COMPLEMENTARY_GLOBS and a new 'validitems'
varflag on IMAGE_FEATURES (so that additional non-package group features
can be added elsewhere.)
Implements [YOCTO #3308].
Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com> Signed-off-by: Saul Wold <sgw@linux.intel.com>
