classes/whitelist: add class to allow whitelisting recipes from a layer

classes/whitelist: add class to allow whitelisting recipes from a layer

Allow restricting recipes brought from a layer to a specified list. This
is similar in operation to blacklist.bbclass, but instead specifies a
per-layer whitelist of recipes (matched by PN or BPN) that are able to
be built from the layer - anything else is skipped. This is potentially
useful where you want to bring in a select set of recipes from a larger
layer e.g. meta-oe.

Implements [YOCTO #8150].

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