MR!495
1. add MESH_EVENT_SCAN_DONE event.
2. add APIs esp_mesh_scan_get_ap_record() and
esp_mesh_scan_get_ap_ie_len() to get scan results.
3. modify API esp_mesh_set_self_organized() by adding parameter
"select_parent".
4. modify API esp_mesh_set_parent() by adding parameter
"parent_mesh_id".
5. if self is set as Root-Fixed, ignore root conflicts.
1. Update libphy.a to v3900 (phy submodule commit: 4c02a1dc5)
2. Update librtc.a to v238 (rct submodule commit: ba66f971)
3. Update wifi lib to include following changes (fa8f8c64)
- add scan_method, sort_method and threshold when invoke esp_wifi_get_config()
- add rssi of each STA in softAP list
- add country information in scan result
- add noise floor in rx pkt
1. fix issues on sending beacon caused by too much tx retries on other packets.
2. modify not to scan if rc exists when connect.
3. modify scan dwell time to default 120ms fo root.
1. fix a bug in initliazing map beacon interval.
2. fix not send MESH_EVENT_ROUTING_TABLE_REMOVE when MESH_NWK_CHILD_DISCONNECTED.
3. fix a bug in returning roots ie len.