Port freertos to run on qemu

1. Freertos source code download

Reference blog:"freertos source code download and directory structure analysis";

2. Compile freertos

2.1. Choose the appropriate Demo

freertos has officially adapted to qemu, so we do not need to do source code level transplantation, we only need to select the appropriate Demo folder.

2.2. Modify Makefile

2.3. Compile

3. Run freertos on qemu

For qemu installation, please refer to the blog:"Qemu Source Code Download and Installation";