bt: call nvs_flash_init in examples, show error if NVS is not initialized

NVS is used to store PHY calibration data, WiFi configuration, and BT
configuration. Previously BT examples did not call nvs_flash_init,
relying on the fact that it is called during PHY init. However PHY init
did not handle possible NVS initialization errors.

This change moves PHY init procedure into the application, and adds
diagnostic messages to BT config management routines if NVS is not
initialized.

examples/bluetooth/gatt_security_server/main/example_ble_sec_gatts_demo.c

@@ -366,6 +366,15 @@ static void gatts_event_handler(esp_gatts_cb_event_t event, esp_gatt_if_t gatts_
 void app_main()
 {
     esp_err_t ret;
+
+    // Initialize NVS.
+    ret = nvs_flash_init();
+    if (ret == ESP_ERR_NVS_NO_FREE_PAGES) {
+        ESP_ERROR_CHECK(nvs_flash_erase());
+        ret = nvs_flash_init();
+    }
+    ESP_ERROR_CHECK( ret );
+
     esp_bt_controller_config_t bt_cfg = BT_CONTROLLER_INIT_CONFIG_DEFAULT();
     ret = esp_bt_controller_init(&bt_cfg);
     if (ret) {