The exti layer now allows drivers to register interrupt callbacks
during board initialization. All details of the driver using a
particular EXTI pin have been removed from the EXTI layer so it
can now be used on any board without board-specific modification.
This includes some nice refinements provided by Mike Smith during
initial review. His original commits have been squashed into this
one.
We do not want to panic in this case as we want the ability to detect if
objects exist at run time
This reverts commit 78e42470d47e987633b049224ff26fbec3aa3444.
