Add example USART driver

Makefile

@@ -5,7 +5,7 @@ CPU = -mcpu=cortex-m4
 THUMB = -mthumb
 
 LDFLAGS = -T linker.ld
-CFLAGS = $(CPU) $(THUMB) -I libc/
+CFLAGS = $(CPU) $(THUMB) -I libc/ -I ./
 
 QEMU = qemu-system-arm
 BOARD = netduino2
@@ -13,7 +13,7 @@ BOARD = netduino2
 # Output file
 TARGET = main.elf
 # Source files
-SRCS = os/main.c os/process.c os/scheduler.c os/alloc.c libc/library.c tasks/tasks.c
+SRCS = os/main.c os/process.c os/scheduler.c os/alloc.c os/delay.c os/driver/usart.c libc/library.c tasks/tasks.c
 # Object files
 OBJS = $(SRCS:.c=.o)