examples: Standardise naming of files, symbols, etc. in examples

* Use "example" in all example function & variable names,
  ie use i2c_example_xxx instead of i2c_xxx for example functions.
  Closes #198 https://github.com/espressif/esp-idf/issues/198
* Mark example functions, etc. static
* Replace uses of "test" & "demo" with "example"
* Split the UART example into two
* Rename "main" example files to end with "_main.c" for disambiguation

examples/peripherals/sigmadelta/main/sigmadelta_example_main.c

@@ -0,0 +1,54 @@
+/* Sigma-delta Example
+
+   This example code is in the Public Domain (or CC0 licensed, at your option.)
+
+   Unless required by applicable law or agreed to in writing, this
+   software is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR
+   CONDITIONS OF ANY KIND, either express or implied.
+*/
+#include <stdio.h>
+#include "freertos/FreeRTOS.h"
+#include "freertos/task.h"
+#include "esp_system.h"
+#include "driver/sigmadelta.h"
+
+/*
+ * This test code intended to configure sigma-delta and set GPIO4 as signal output pin.
+ * If you connect this GPIO4 with an LED, you will see the LED blinking slowly.
+ */
+
+/**
+ * @brief Sigma-delta initialization.
+ */
+static void sigmadelta_example_init(void)
+{
+   sigmadelta_config_t sigmadelta_cfg = {
+       /* Sigma-delta channel0*/
+       .channel = SIGMADELTA_CHANNEL_0,
+       /* Sigma-delta set duty 10*/
+       .sigmadelta_duty = 10,
+       /* Set prescale 30 */
+       .sigmadelta_prescale = 80,
+       /*select GPIO4 as output_io */
+       .sigmadelta_gpio = 4,
+     };
+   sigmadelta_config(&sigmadelta_cfg);
+}
+
+/**
+ * @brief Sigma-delta test to change duty of output signal.
+ */
+void app_main()
+{
+    sigmadelta_example_init();
+    int8_t duty = 0;
+    int inc = 1;
+    while(1) {
+        sigmadelta_set_duty(SIGMADELTA_CHANNEL_0, duty);
+        /*by changing delay time, you can change the blink frequency of LED. */
+        vTaskDelay(10 / portTICK_PERIOD_MS);
+
+        duty += inc;
+        if(duty == 127 || duty == -127) inc = (-1) * inc;
+    }
+}