Makefile: Create variable SRCS to list needed source files
Makefile: Add USE_ASM_OPTI_FUNC variable
Move until functions to util.c such as strcpy, strlen..etc
Add compile flag to select optimized function or not at compile time
Makefile: Replace depend component to $(SRCS)
Makefile: Clean up syscall.s. It is derived from syscall.S
Implement util functions when optimization function is disabled