ling1995:
My goal is to do develop some boards that can support STM32F4,STM32F7 and Discovery.
STM32F4 is already done:
You could probably use the above project as well as the STM32duino packages:
as a reference for STM32F7 if nobody has done that one yet. I don't know exactly what you mean by Discovery, those seem to have various MCU models so some of the Discovery boards may already be supported by Arduino_STM32 and STM32_duino.
You will also find the official hardware specification useful to your project:
I think adding support to the Arduino IDE for any additional hardware is a very valuable contribution. If you do end up taking on such a project feel free to check back in here on the forum if you run into any problems, there are some people here who are very knowledgeable of the general process, if not with your specific target hardware. I'm happy to help if I can. I have quite a bit of experience with adding Boards Manager installation support if you decide that's desirable.