esp8266: Add alternative event loop implementation.

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.