Fix some compilation warnings and add esp-idf platform for experiment (#454)

And fix some code indent issues.

product-mini/platforms/esp-idf/CMakeLists.txt

@@ -0,0 +1,55 @@
+# Copyright (C) 2019 Intel Corporation.  All rights reserved.
+# SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+# from ESP-IDF 4.0 examples/build_system/cmake/idf_as_lib
+cmake_minimum_required(VERSION 3.5)
+
+project(wamr_esp_idf C)
+
+enable_language (ASM)
+
+# Include for ESP-IDF build system functions
+include($ENV{IDF_PATH}/tools/cmake/idf.cmake)
+
+# Create idf::esp32 and idf::freertos static libraries
+idf_build_process(esp32
+                  # try and trim the build; additional components
+                  # will be included as needed based on dependency tree
+                  #
+                  # although esptool_py does not generate static library,
+                  # processing the component is needed for flashing related
+                  # targets and file generation
+                  COMPONENTS esp32 freertos esptool_py
+                  SDKCONFIG ${CMAKE_BINARY_DIR}/sdkconfig
+                  BUILD_DIR ${CMAKE_BINARY_DIR})
+
+include_directories(build/config
+                    xtensa/include
+                    $ENV{IDF_PATH}/components/esp32/include
+                    $ENV{IDF_PATH}/components/esp_common/include
+                    $ENV{IDF_PATH}/components/esp_rom/include
+                    $ENV{IDF_PATH}/components/freertos/include
+                    $ENV{IDF_PATH}/components/heap/include
+                    $ENV{IDF_PATH}/components/soc/esp32/include
+                    $ENV{IDF_PATH}/components/xtensa/include
+                    $ENV{IDF_PATH}/components/xtensa/esp32/include)
+
+set(WAMR_BUILD_PLATFORM "esp-idf")
+set(WAMR_BUILD_TARGET "XTENSA")
+set(WAMR_BUILD_INTERP 1)
+set(WAMR_BUILD_FAST_INTERP 1)
+set(WAMR_BUILD_AOT 1)
+set(WAMR_BUILD_LIBC_BUILTIN 1)
+set(WAMR_BUILD_LIBC_WASI 0)
+
+set(WAMR_ROOT_DIR ${CMAKE_CURRENT_SOURCE_DIR}/../../..)
+
+include(${WAMR_ROOT_DIR}/build-scripts/runtime_lib.cmake)
+add_library(vmlib ${WAMR_RUNTIME_LIB_SOURCE})
+
+set(elf_file ${CMAKE_PROJECT_NAME}.elf)
+add_executable(${elf_file} main.c iwasm_main.c)
+
+# Link the static libraries to the executable
+target_link_libraries(${elf_file} idf::esp32 idf::freertos idf::spi_flash vmlib)
+