WCH CH569W-R0-1v0 evt board bsp port, first version

dev/test under Ubuntu 20.04
toolchain from MounRiver_Studio_Community_Linux_x64_V120

tested drivers : SysTick, gpio, gpio interrupt, uart1 (RX interrupt, TX polling)

libcpu/risc-v/SConscript :
group includes rtconfig.CPU only if folder exists

libcpu/risc-v/common/cpuport.c/rt_hw_context_switch_interrupt() :
make it RT_WEAK for customization