add RIOT-OS support to WAMR (#425)

* add RIOT platform

see riot-os.org

* add simple RIOT example

product-mini/platforms/riot/CMakeLists.txt

@@ -0,0 +1,59 @@
+# Copyright (C) 2019 Intel Corporation.  All rights reserved.
+# Copyright (C) 2020 TU Bergakademie Freiberg Karl Fessel
+# SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+cmake_minimum_required(VERSION 3.8.2)
+
+set(CMAKE_TRY_COMPILE_TARGET_TYPE "STATIC_LIBRARY")
+
+project(NONE)
+
+enable_language (ASM)
+
+set (WAMR_BUILD_PLATFORM "riot")
+
+# Build as X86_32 by default, change to "AARCH64[sub]", "ARM[sub]", "THUMB[sub]", "MIPS" or "XTENSA"
+# if we want to support arm, thumb, mips or xtensa
+
+
+if (NOT DEFINED WAMR_BUILD_TARGET)
+  set (WAMR_BUILD_TARGET "X86_32")
+endif ()
+
+if (NOT DEFINED WAMR_BUILD_INTERP)
+  # Enable Interpreter by default
+  set (WAMR_BUILD_INTERP 1)
+endif ()
+
+if (NOT DEFINED WAMR_BUILD_AOT)
+  # Disable AOT by default.
+  set (WAMR_BUILD_AOT 0)
+endif ()
+
+if (NOT DEFINED WAMR_BUILD_LIBC_BUILTIN)
+  # Enable libc builtin support by default
+  set (WAMR_BUILD_LIBC_BUILTIN 1)
+endif ()
+
+if (NOT DEFINED WAMR_BUILD_LIBC_WASI)
+  # Disable libc wasi support by default
+  set (WAMR_BUILD_LIBC_WASI 0)
+endif ()
+
+if (NOT DEFINED WAMR_ROOT_DIR)
+  # this assumption is true if this file is copied to WAMR_ROOT
+  set (WAMR_ROOT_DIR ${CMAKE_CURRENT_SOURCE_DIR})
+endif ()
+
+
+include (${WAMR_ROOT_DIR}/build-scripts/runtime_lib.cmake)
+
+# need includes from RIOT prepare them as a cmake list
+string(REGEX MATCHALL "([^\ ]+\ |[^\ ]+$)" RIOT_INCLUDES_LIST "${RIOT_INCLUDES}")
+
+include_directories(SYSTEM ${RIOT_INCLUDES_LIST})
+
+# target_sources( ${WAMR_RUNTIME_LIB_SOURCE} )
+# executable linking is done by RIOT build system
+
+add_library( wamr ${WAMR_RUNTIME_LIB_SOURCE})