add regular scheduled functions, now also callable on yield() (#6039)

* add regular scheduled functions, now also callable on `yield()`

added bool schedule_function_us(std::function<bool(void)> fn, uint32_t repeat_us)
lambda must return true to be not removed from the schedule function list
if repeat_us is 0, then the function is called only once.

Legacy schedule_function() is preserved

This addition allows network drivers like ethernet chips on lwIP to be regularly called
- even if some user code loops on receiving data without getting out from main loop
  (callable from yield())
- without the need to call the driver handling function
  (transparent)

This may be also applicable with common libraries (mDNS, Webserver, )

tools/sdk/include/c_types.h

@@ -93,6 +93,9 @@ typedef enum {
 #define ICACHE_RAM_ATTR
 #endif /* ICACHE_FLASH */
 
+// counterpart https://github.com/espressif/arduino-esp32/blob/master/cores/esp32/esp8266-compat.h
+#define IRAM_ATTR ICACHE_RAM_ATTR
+
 #define STORE_ATTR __attribute__((aligned(4)))
 
 #ifndef __cplusplus