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.
diff --git a/esp8266/ets_alt_task.h b/esp8266/ets_alt_task.h
new file mode 100644
index 0000000..c423995
--- /dev/null
+++ b/esp8266/ets_alt_task.h
@@ -0,0 +1 @@
+bool ets_loop_iter(void);