Corrected thread tasklet during interface disconnect/connect

Added check for valid interface ID to network scan timer start so that it
is not triggered after disconnect. Added variable for mac api so that it
is not tried to set again after re-connecting.

Author: Mika Leppänen

features/nanostack/mbed-mesh-api/source/thread_tasklet.c

@@ -84,6 +84,7 @@ typedef struct {
 
 /* Tasklet data */
 static thread_tasklet_data_str_t *thread_tasklet_data_ptr = NULL;
+static mac_api_t *mac_api = NULL;
 static device_configuration_s device_configuration;
 
 /* private function prototypes */
@@ -216,7 +217,8 @@ void thread_tasklet_parse_network_event(arm_event_s *event)
             break;
     }
 
-    if (thread_tasklet_data_ptr->tasklet_state != TASKLET_STATE_BOOTSTRAP_READY) {
+    if (thread_tasklet_data_ptr->tasklet_state != TASKLET_STATE_BOOTSTRAP_READY &&
+        thread_tasklet_data_ptr->nwk_if_id != INVALID_INTERFACE_ID) {
         // Set 5s timer for a new network scan
         eventOS_event_timer_request(TIMER_EVENT_START_BOOTSTRAP,
                                     ARM_LIB_SYSTEM_TIMER_EVENT,
@@ -465,8 +467,10 @@ int8_t thread_tasklet_network_init(int8_t device_id)
     storage_sizes.key_description_table_size = 6;
     storage_sizes.key_lookup_size = 1;
     storage_sizes.key_usage_size = 3;
-    mac_api_t *api = ns_sw_mac_create(device_id, &storage_sizes);
-    return arm_nwk_interface_lowpan_init(api, INTERFACE_NAME);
+    if (!mac_api) {
+        mac_api = ns_sw_mac_create(device_id, &storage_sizes);
+    }
+    return arm_nwk_interface_lowpan_init(mac_api, INTERFACE_NAME);
 }
 
 void thread_tasklet_device_eui64_set(const uint8_t *eui64)