Added USB example for stm32f4.

Moved clock enable for OTG_FS back into applications.  This and gpio config
are the only differences between f107 and f2xx/f4xx.

lib/stm32/f4/Makefile

@@ -30,6 +30,7 @@ CFLAGS		= -Os -g -Wall -Wextra -I../../../include -fno-common \
 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 \
 
 VPATH += ../../usb:../