packagegroup-base: Add DISTRO_FEATURES and groups for Thread feature

Thread connectivity is a feature that can come as a full border router
or simple client flavour. They both drag in different dependencies
through the matching packagegroup. Demonstrate the use in our gateway
blueprint.

Fixes: https://git.ostc-eu.org/OSTC/planning/blueprints/-/issues/7



Signed-off-by: Stefan Schmidt <stefan.schmidt@huawei.com>