Add malloc and free instead directly calling pvPortMalloc and vPortFree
Add Makefile target "screen"
Fix the serial.getc stuck issue
Rename memory.h and memory.c to stdlib.h and stdlib.c
Update .gitignore
Move out some C Std functions from rs232.c and rs232.h
Support printf to the serial (serial.printf)
Rename RS232 related code's name to Serial
Merge pull request #58 from shengwen1997/master

Sort the C Std functions