[STM32F3] Split out F3 specific exti definitions.
[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.
[STM32F3] Split timer common f24 into f234 and f24.