add porting codes of rt-thread (#494)

SConscript

@@ -0,0 +1,33 @@
+#
+# Copyright (c) 2021, RT-Thread Development Team
+#
+# SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+#
+
+# for module compiling
+import os
+
+from building import *
+
+objs = []
+cwd  = GetCurrentDir()
+list = os.listdir(cwd)
+
+if GetDepend(['PKG_USING_WAMR']):
+    wamr_entry_sconscript = os.path.join(cwd, "product-mini", "platforms", "rt-thread", 'SConscript')
+
+    if os.path.isfile(wamr_entry_sconscript):
+        objs = objs + SConscript(wamr_entry_sconscript)
+    else:
+        print("[WAMR] entry script wrong:", wamr_entry_sconscript)
+        Return('objs')
+
+    wamr_runlib_sconsript = os.path.join(cwd, "build-scripts", 'SConscript')
+
+    if os.path.isfile(wamr_runlib_sconsript):
+        objs = objs + SConscript(wamr_runlib_sconsript)
+    else:
+        print("[WAMR] runtime lib script wrong:", wamr_runlib_sconsript)
+
+Return('objs')
+