Adds the model extension concept to the access layer, as described in
the Mesh Profile Specification, Section 2.3.6. Extensions are
implemented as a tree, using two pointers in each model:
The extends pointer points to the first extended model, and the next
pointer points to the next sibling or (if the NEXT_IS_PARENT flag is
set) the parent model in the tree, forming a cyclical "Left-child
right-sibling" (LCRS) tree. The tree root can be obtained by calling
bt_mesh_model_root_get(), and the extended models can be walked by
calling bt_mesh_model_tree_walk().
According to the Mesh Profile Specification Section 4.2.3, all models in
the same extension tree share one subscription list per element. This is
implemented by walking the model's extension tree, and pooling the
subscription lists of all models in the same element into one. If the
config server adds a subscription to a model, it may be stored in any of
the model tree's models' subscription lists. No two models in the same
extension tree and element will have duplicate groups listed. This
allows us to increase extended models' capacity for subscriptions
significantly.
Signed-off-by: Trond Einar Snekvik <Trond.Einar.Snekvik@nordicsemi.no>
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