Rename sample app to "hello_world"

examples/hello_world/Makefile

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+PROJECT_NAME := hello_world
+
+include $(IDF_PATH)/make/project.mk

examples/hello_world/components/ttn-esp32

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+../../../../ttn-esp32

examples/hello_world/main/main.cpp

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+/*******************************************************************************
+ * Copyright (c) 2018 Manuel Bleichenbacher
+ * 
+ * All rights reserved. This program and the accompanying materials
+ * are made available under the terms of the Eclipse Public License v1.0
+ * which accompanies this distribution, and is available at
+ * http://www.eclipse.org/legal/epl-v10.html
+ *
+ * Sample program showing how to send a test message every 30 second.
+ *******************************************************************************/
+
+#include "freertos/FreeRTOS.h"
+#include "esp_event.h"
+
+#include "TheThingsNetwork.h"
+
+// NOTE:
+// The LoRaWAN frequency and the radio chip must be configured by running 'make menuconfig'.
+// Go to Components / The Things Network, select the appropriate values and save.
+
+// Copy the below two lines from bottom of your device's overview page in the TTN console
+const char *appEui = "????????????????";
+const char *appKey = "????????????????????????????????";
+
+// Copy the below hex string from the "Device EUI" field on the same pag.
+const char *devEui = "????????????????";;
+
+
+static TheThingsNetwork ttn;
+
+const unsigned TX_INTERVAL = 30;
+static uint8_t msgData[] = "Hello, world";
+
+
+void send_messages(void* pvParameter)
+{
+    while (1) {
+        printf("Sending message...\n");
+        ttn_response_t res = ttn.sendBytes(msgData, sizeof(msgData) - 1);
+        if (res == TTN_SUCCESSFUL_TRANSMISSION)
+            printf("Message sent.\n");
+        else
+            printf("Message transmission failed.\n");
+
+        vTaskDelay(TX_INTERVAL * 1000 / portTICK_PERIOD_MS);
+    }
+}
+
+extern "C" void app_main(void)
+{
+    gpio_install_isr_service(ESP_INTR_FLAG_IRAM);
+
+    // Initialize SPI bus
+    spi_bus_config_t spi_bus_config;
+    spi_bus_config.miso_io_num = 19;
+    spi_bus_config.mosi_io_num = 27;
+    spi_bus_config.sclk_io_num = 5;
+    spi_bus_config.quadwp_io_num = -1;
+    spi_bus_config.quadhd_io_num = -1;
+    spi_bus_config.max_transfer_sz = 0;
+
+    esp_err_t ret = spi_bus_initialize(HSPI_HOST, &spi_bus_config, 1);
+    assert(ret == ESP_OK);
+
+    ttn.configurePins(HSPI_HOST, 18, TTN_NOT_CONNECTED, 14, 26, 33);
+
+    ttn.provision(appEui, appKey, devEui);
+
+    printf("Joining...\n");
+    ttn.join();
+    printf("Joined.\n");
+
+    xTaskCreate(send_messages, "send_messages", 1024 * 4, (void* )0, 3, NULL);
+}