Integrate WASI-NN into WAMR (#1521)

Initial integration of WASI-NN based on #1225:
- Implement the library core/iwasm/libraries/wasi-nn
- Support TensorFlow, CPU, F32 at the first stage
- Add cmake variable `-DWAMR_BUILD_WASI_NN`
- Add test case based on Docker image and update document

Refer to #1573

core/iwasm/libraries/wasi-nn/test/build.sh

@@ -0,0 +1,20 @@
+# Copyright (C) 2019 Intel Corporation.  All rights reserved.
+# SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+# WASM application that uses WASI-NN
+
+/opt/wasi-sdk/bin/clang \
+    -Wl,--allow-undefined \
+    -Wl,--strip-all,--no-entry \
+    --sysroot=/opt/wasi-sdk/share/wasi-sysroot \
+    -I/home/wamr/core/iwasm/libraries/wasi-nn \
+    -o test_tensorflow.wasm test_tensorflow.c
+
+# TFLite models to use in the tests
+
+cd models
+python3 average.py
+python3 max.py
+python3 mult_dimension.py
+python3 mult_outputs.py
+python3 sum.py