Bluetooth: Mesh: Added support for application access to mesh messages

Added bt_mesh_msg_send() which can be used by the application to
directly send model layer messages without local instantiation of
related models. Also added bt_mesh_msg_cb_set() which allows the
application to recieve mesh model layer messages without local
instantiation of related models.

Added bt_mesh_has_addr() which returns a bool. For unicast addresses,
this returns whether or not bt_mesh_elem_find() was successfull. If the
above mentioned bt_mesh_msg_cb_set() has been used by the application to
set a message callback, this returns true so that the stack attempts to
push every model message up to the application via the callback. If no
callback has been set, group addresses are searched to see if the stack
should pass the message up the stack to an instantiated model.

These changes allow applications that do not or can not instantiate
models to interface with models in a mesh network. This is applicable to
applications which act as a Bluetooth mesh gateway, sniffer, debugger,
network monitoring, non-mesh relay/extender, etc.

In app_keys.c friend.c net.c bt_mesh_elem_find() is used only to
determine the existance of an address. The full return value of
bt_mesh_elem_find() is unecessary and so was replaced by the above
mentioned bt_mesh_has_addr() function in these instances.

Simplified bt_mesh_elem_find() by removing the search through group
address. Since the above mentioned bt_mesh_has_addr() function handles
instances where group addresses must be searched, it was no longer
necessary to preform this search in this function.

Signed-off-by: Bud Wandinger <bud@budkoembedded.ca>

subsys/bluetooth/mesh/app_keys.c

@@ -460,7 +460,7 @@ int bt_mesh_keys_resolve(struct bt_mesh_msg_ctx *ctx,
 		}
 
 		if (ctx->app_idx == BT_MESH_KEY_DEV_REMOTE &&
-		    !bt_mesh_elem_find(ctx->addr)) {
+		    !bt_mesh_has_addr(ctx->addr)) {
 			struct bt_mesh_cdb_node *node;
 
 			if (!IS_ENABLED(CONFIG_BT_MESH_CDB)) {