Arduino board support package for LinkIt 7697

This is the source code for Arduino Add-On package for LinkIt 7697. To install the board support package, visit this resource page.

The source code is an add-on to LinkIt SDK v4.x. To build the board support package, you need following prerequisites:

Build Prerequisites

You need a working LinkIt SDK v4.6 public version. To obtain public version of LinkIt SDK v4.6, visit https://docs.labs.mediatek.com/resource/mt7687-mt7697/en/downloads
You must build this package on a Linux machine with make and perl installed.

Copy LinkIt SDK v4 public version into the source repo. We want to keep existing files, since this is an add-on package. We assume the SDK is unpacked in Arduino-Add-On-for-LinkIt-SDK.
```
git clone --depth 1 https://github.com/MediaTek-Labs/Arduino-Add-On-for-LinkIt-SDK.git
tar --keep-old-files -zxf LinkIt_SDK_V4.6.0.tar.gz -C Arduino-Add-On-for-LinkIt-SDK
```
Use following command to build the board support package:
```
mkdir out
./mk_ide_pkg.pl -o out -v 1.0.0
```
Where the version number 1.0.0 can be assigned by yourself. The resulting packages are located in the out directory. You can also refer to the Travis CI config file .travis.yml as an example on the building process.

This package is based on Archermind's LinkIt 7687 Arduino package, with some modifications for MT7697 and LinkIt 7697 board.

There are 2 major parts in this add-on package:

a middleware module of LinkIt SDK that provides an Arduino porting layer. Most of the porting layer codes are availble in the BSP(board supporting package) as source code format. The porting layer is located in /middleware/third_party/arduino/hardware/arduino/mt7697/.
a project for LinkIt 7697 HDK that builds all the required hardware drivers, FreeRTOS kernel, Wi-Fi and Bluetooth frameworks into a library. The binary library is then copied to the final BSP package. The path to the project is /project/linkit7697_hdk/apps/arduino/arduino_lib