Add initial SPI code.

For now, add the following basic SPI functions:

 - spi_init_master()
 - spi_write()
 - spi_read()

This is incomplete and untested, yet.

Also, add some more SPI bit definition macros and comments.

lib/Makefile

@@ -27,7 +27,7 @@ CFLAGS		= -Os -g -Wall -Wextra -I../include -fno-common \
 		  -mcpu=cortex-m3 -mthumb -Wstrict-prototypes
 # ARFLAGS	= rcsv
 ARFLAGS		= rcs
-OBJS		= rcc.o gpio.o usart.o adc.o
+OBJS		= rcc.o gpio.o usart.o adc.o spi.o
 
 # Be silent per default, but 'make V=1' will show all compiler calls.
 ifneq ($(V),1)