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/wasi_nn.cmake

@@ -0,0 +1,10 @@
+# Copyright (C) 2019 Intel Corporation.  All rights reserved.
+# SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+set (WASI_NN_DIR ${CMAKE_CURRENT_LIST_DIR})
+
+add_definitions (-DWASM_ENABLE_WASI_NN=1)
+
+set (LIBC_WASI_NN_SOURCE ${WASI_NN_DIR}/wasi_nn_native.c ${WASI_NN_DIR}/wasi_nn_tensorflow.cpp)
+
+set (TENSORFLOW_LIB tensorflow-lite)