ets_loop_iter processes pending tasks, and tasks are considered lower
priority than interrupts, so tasks shouldn't be processed if interrupts
are disabled.
This implementation provides the same interface and uses the same
datastructures as used by BootROM, i.e. is a drop-in replacement for it.
But it offers one advantage: it allows to run single iteration of
event-pumping loop.
Original BootROM function are renamed, prefixed with underscore. There's
a switch which allows to use forward calls to them, for compatibility
testing.
The implementation also includes workarounds for hardware timer handler,
and these workarounds may be SDK version specific.
