Porting RT-Thread to AnnikaSoC

This paper implements porting RT-Thread OS to RISC-V based AnnikaSoC MCU, whose work includes modifying linker script, writing startup file, adapting interrupt and exception interface, porting UART and Timer drivers, adapting thread context switch interface and BSP entry. We did variety of tests after porting, whose result shows that all parts of RT-Thread our porting work involves can work properly.