Make waveform generator a NMI to run always

Make the waveform generator an NMI using the same code as in 2.4.0.
Making it NMI will ensure it runs even when interrupts are disabled.

Fixes esp8266#5568

cores/esp8266/core_esp8266_waveform.c

@@ -38,6 +38,7 @@
 */
 
 #include <Arduino.h>
+#include "ets_sys.h"
 #include "core_esp8266_waveform.h"
 
 // Need speed, not size, here
@@ -130,15 +131,15 @@ static uint32_t lastCycleCount = 0; // Last ESP cycle counter on running the int
 
 static void initTimer() {
   timer1_disable();
-  timer1_isr_init();
-  timer1_attachInterrupt(timer1Interrupt);
+  ETS_FRC_TIMER1_INTR_ATTACH(NULL, NULL);
+  ETS_FRC_TIMER1_NMI_INTR_ATTACH(timer1Interrupt);
   lastCycleCount = GetCycleCount();
   timer1_enable(TIM_DIV1, TIM_EDGE, TIM_SINGLE);
   timerRunning = true;
 }
 
 static void ICACHE_RAM_ATTR deinitTimer() {
-  timer1_attachInterrupt(NULL);
+  ETS_FRC_TIMER1_NMI_INTR_ATTACH(NULL);
   timer1_disable();
   timer1_isr_init();
   timerRunning = false;