Update license for RT-Thread kernel.
1. Move struct stat to libc_stat.h; 2. Defined a new FD_SET macros in libc_fdset.h.
1. Remove components.h file; 2. Add libc_* files for standard libc definitions; 3. Add rtdbg.h file for simple debug log; 4. Add single list implementation; 5. Change the 'rt_uint8_t' type of cmd to 'int'.