[esp] Add ESP-IDF cmake example.

esp/cpp/04_esp-idf-arduino/main/CMakeLists.txt

@@ -0,0 +1,4 @@
+idf_component_register(
+    SRCS "main.cpp"
+    INCLUDE_DIRS "."
+)

esp/cpp/04_esp-idf-arduino/main/idf_component.yml

@@ -0,0 +1,17 @@
+## IDF Component Manager Manifest File
+dependencies:
+  ## Required IDF version
+  idf:
+    version: '>=4.1.0'
+  # # Put list of dependencies here
+  # # For components maintained by Espressif:
+  # component: "~1.0.0"
+  # # For 3rd party components:
+  # username/component: ">=1.0.0,<2.0.0"
+  # username2/component2:
+  #   version: "~1.0.0"
+  #   # For transient dependencies `public` flag can be set.
+  #   # `public` flag doesn't have an effect dependencies of the `main` component.
+  #   # All dependencies of `main` are public by default.
+  #   public: true
+  espressif/arduino-esp32: ^3.1.1

esp/cpp/04_esp-idf-arduino/main/main.cpp

@@ -0,0 +1,10 @@
+#include "Arduino.h"
+
+void setup() {
+  Serial.begin(115200);
+}
+
+void loop() {
+  Serial.println("Hello world!");
+  delay(1000);
+}