Add assert macros to aid in debugging.

Adds libopencm3/cm3/assert.h header that provides assertion check macros
similar to those provided by the standard C library.

Thanks to Nicolas Schodet for help.

lib/stm32/f4/Makefile

@@ -31,7 +31,8 @@ ARFLAGS		= rcs
 OBJS		= vector.o rcc.o gpio.o usart.o spi.o flash.o nvic.o \
 		  i2c.o systick.o exti.o scb.o pwr.o timer.o \
 		  usb.o usb_standard.o usb_control.o usb_f107.o \
+		  assert.o
 
-VPATH += ../../usb:../
+VPATH += ../../usb:../:../../cm3
 
 include ../../Makefile.include