EXTINT: Add API for external interrupts.

The new API allows registering efficient ChibiOS EXT callback tha
will be called from an ISR context.

Another way is to register an event callback that is called in the
event loop thread context.

ChibiOS EXT phepripheral is automatically started and stopped.
main-test: Use button presses.

Use the new extint API to get events about button state changes.
lis302dl: Update to use new extint API and INT2.

INT1 collides with USER_BUTTON so they cannot use separate interrupts
thus changing lis302dl to use INT2 instead.