Add esp-idf.gitignore (#3150)

* Add esp-idf.gitignore

From https://github.com/espressif/esp-idf-template/blob/master/.gitignore

* Add header comments to esp-idf.gitignore

community/embedded/esp-idf.gitignore

@@ -0,0 +1,6 @@
+# gitignore template for esp-idf, the official development framework for ESP32
+# https://github.com/espressif/esp-idf
+
+build/
+sdkconfig
+sdkconfig.old