simulator: add createdef.py to generate def file for VS

Now we can get rid off Mingw to create def file. To create def file, run:

    scons --def

Then, you should run 

    scons -j4 

to create rtthread.dll rtthread.lib and rtthread-win32.exe 

Then, cd simulator/testdll
run 

    scons --app=snake

This will create snake.dll in snake/build

Run rtthread_win32.exe, In finsh, run 

     exec("/testdll/snake/build/snake.dll")