1. add custom POSIX support as submodule
2. add FreeRTOS and POSIX CMake build support
3. add FreeRTOS port-related code
4. fix mm support in FreeRTOS
5. modify FreeRTOS example
TODO:
1. D0 interrupt support
2. D0 privilege support
* use nuttx libc, disable system libc
* use tlsf as default
* update lhal flash driver
* add example readme
* add flash ini for new flash tool
* add fw header for new flash tool