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.

components/nvs_flash/src/nvs_api.cpp

@@ -86,6 +86,17 @@ extern "C" esp_err_t nvs_flash_init(void)
     return nvs_flash_init_custom(partition->address / SPI_FLASH_SEC_SIZE,
             partition->size / SPI_FLASH_SEC_SIZE);
 }
+
+extern "C" esp_err_t nvs_flash_erase()
+{
+    const esp_partition_t* partition = esp_partition_find_first(
+            ESP_PARTITION_TYPE_DATA, ESP_PARTITION_SUBTYPE_DATA_NVS, NULL);
+    if (partition == NULL) {
+        return ESP_ERR_NOT_FOUND;
+    }
+
+    return esp_partition_erase_range(partition, 0, partition->size);
+}
 #endif
 
 static esp_err_t nvs_find_ns_handle(nvs_handle handle, HandleEntry& entry)