Move interrupt busy detection from dda_clock() to clock tick ISR.

The reason is, interrupts can't be re-entered on ARM, so we need
a different mechanism.

dda.c

@@ -749,7 +749,6 @@ void dda_step(DDA *dda) {
   accurate curves!
 */
 void dda_clock() {
-  static volatile uint8_t busy = 0;
   DDA *dda;
   static DDA *last_dda = NULL;
   uint8_t endstop_trigger = 0;
@@ -769,13 +768,6 @@ void dda_clock() {
   if (dda == NULL)
     return;
 
-  // Lengthy calculations ahead!
-  // Make sure we didn't re-enter, then allow nested interrupts.
-  if (busy)
-    return;
-  busy = 1;
-  sei();
-
   // Caution: we mangle step counters here without locking interrupts. This
   //          means, we trust dda isn't changed behind our back, which could
   //          in principle (but rarely) happen if endstops are checked not as
@@ -923,9 +915,6 @@ void dda_clock() {
       ATOMIC_END
     }
   #endif
-
-  cli(); // Compensate sei() above.
-  busy = 0;
 }
 
 /// update global current_position struct

timer-avr.c

@@ -41,11 +41,25 @@ uint32_t	step_extra_time = 0;
   Comparator B is the system clock, happens every TICK_TIME.
 */
 ISR(TIMER1_COMPB_vect) {
+  static volatile uint8_t busy = 0;
+
 	// set output compare register to the next clock tick
 	OCR1B = (OCR1B + TICK_TIME) & 0xFFFF;
 
   clock_tick();
-  dda_clock();
+
+  /**
+    Lengthy calculations ahead! Make sure we didn't re-enter, then allow
+    nested interrupts.
+  */
+  if ( ! busy) {
+    busy = 1;
+    sei();
+
+    dda_clock();
+
+    busy = 0;
+  }
 }
 
 #ifdef	MOTHERBOARD