map-agent: beacon metrics query & response

Initial set of patches with beacon metrics query/response functionality. This contains functionality as per easy mesh specification - section 10.3.3. The basic functionality now works with some things mocked. Example call: ubus call map.agent bcn_metrics_query '{"agent":"44:d4:37:42:47:b9", "sta":"44:d4:37:4d:84:83", "bssid":"44:d4:37:42:47:bf", "ssid":"MAP-$BASEMAC-5GHz", "channel":255, "channel_report":[{"opclass":81,"channels": [1, 6, 13]}, {"opclass":82, "channels": [1, 6, 13]}], "reporting_detail":1, "request_element": [7, 33]}'