linc80: continued work on proper IM2 support

linc80: continued work on proper IM2 support

Add IM2 driven serial interrupts with proper queues. We don't yet do all the
status and error bits right.

This enables IM2 serial but we don't gain much from it because we have yet
to tackle the core changes needed to leave other interrupt lines running for
most cases where we today DI, and to soft count, skip re-entry of the
interrupt_handler/timer proper.