[STM32] Removed exti.c from f1 and made global exti.c usable.

Because most changes are internal to a function in exti.c I am leaving
the macro checks in there, otherwise we would end up with a bunch of
code duplication if we used the dispatch system used otherwise
throughout the stm32 part of the library. I bet it could be split up
into more granular functions resulting in more generic code that we
could run through the dispatch system. But I am leaving that as an
excersize for later.

lib/stm32/l1/Makefile

@@ -32,7 +32,7 @@ CFLAGS		= -Os -g \
 		  -ffunction-sections -fdata-sections -MD -DSTM32L1
 # ARFLAGS	= rcsv
 ARFLAGS		= rcs
-OBJS		= crc.o desig.o exti2.o flash.o rcc.o usart.o
+OBJS		= crc.o desig.o exti.o flash.o rcc.o usart.o
 OBJS		+= crc_common_all.o dac_common_all.o
 OBJS		+= dma_common_f13.o
 OBJS		+= gpio_common_all.o gpio_common_f234.o