- Reduce available SPL levels for hardware devices to none, vm, sched, high.
- Acquire kernel_lock only for interrupts at IPL_VM.
- Implement threaded soft interrupts.
- Move board-specific PCI/GPIO initialisation to its rightful place.
- Handle clearing down latched GPIO interrupts in a board-independent way.
- Use MI com(4) driver for on-chip UARTs.
- Misc. tidying up.
Tested on IXDP425.