Bluetooth: Mesh: Introduce public LPN APIs for better control

Many apps, the mesh shell included (due to PTS test requirements)
benefit from exposing LPN state and polling outside of the stack.
Introduce new APIs for these, and add code to the mesh shell module to
take advantage of them.

Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>

include/bluetooth/mesh/main.h

@@ -278,6 +278,24 @@ int bt_mesh_provision(const u8_t net_key[16], u16_t net_idx,
  */
 int bt_mesh_lpn_set(bool enable);
 
+/** @brief Send out a Friend Poll message.
+ *
+ *  Send a Friend Poll message to the Friend of this node. If there is no
+ *  established Friendship the function will return an error.
+ *
+ *  @return Zero on success or (negative) error code otherwise.
+ */
+int bt_mesh_lpn_poll(void);
+
+/** @brief Register a callback for Friendship changes.
+ *
+ *  Registers a callback that will be called whenever Friendship gets
+ *  established or is lost.
+ *
+ *  @param cb Function to call when the Friendship status changes.
+ */
+void bt_mesh_lpn_set_cb(void (*cb)(u16_t friend_addr, bool established));
+
 /**
  * @}
  */