ESP IDF fixes (#927)

Various fixes and beautifications coordinated with @1c3t3a,
fixes 2 of the 3 all remaining issues from #892:
- enable to os_mmap executable memory
- fix os_malloc/os_realloc/os_free issues
- implement os_thread_get_stack_boundary
- add build scripts to include with esp-idf to use wamr as
  an ESP-IDF component
- update sample and document

build-scripts/esp-idf/README.md

@@ -0,0 +1,31 @@
+# wasm-micro-runtime as ESP-IDF component
+
+You can build an ESP-IDF project with wasm-micro-runtime as a component:
+
+- Make sure you have the ESP-IDF properly installed and setup
+- In particular have the following paths set:
+  - `WAMR_PATH` to point to your wasm-micro-runtime repository
+  - `IDF_PATH` to point to your ESP-IDF
+  - `source $IDF_PATH/export.sh`
+- Create a new project, e.g.: `idf.py create-project wamr-hello`
+- In the newly created project folder edit the `CMakeList.txt`:
+
+  ```
+  cmake_minimum_required(VERSION 3.5)
+
+  include($ENV{IDF_PATH}/tools/cmake/project.cmake)
+
+  set (COMPONENTS ${IDF_TARGET} main freertos esptool_py wamr)
+
+  list(APPEND EXTRA_COMPONENT_DIRS "$ENV{WAMR_PATH}/build-scripts/esp-idf")
+
+  project(wamr-hello)
+  ```
+- Develop your project in it's `main` component folder.
+
+You can find an example [here](../../product-mini/platforms/esp-idf).
+
+- Set target platform: `idf.py set-target esp32c3`
+- Build: `idf.py build`
+- Flash: `idf.py flash`
+- Check the output: `idf.py monitor`