Fix 1 millisecond backwards time leap in time measured by micros()

This race condition caused periodic flight instability when micros() was
called precisely on a 1000 nanosecond boundary.

src/main/drivers/system.c

@@ -99,6 +99,12 @@ uint32_t micros(void)
     do {
         ms = sysTickUptime;
         cycle_cnt = SysTick->VAL;
+
+        /*
+         * If the SysTick timer expired during the previous instruction, we need to give it a little time for that
+         * interrupt to be delivered before we can recheck sysTickUptime:
+         */
+        asm volatile("\tnop\n");
     } while (ms != sysTickUptime);
     return (ms * 1000) + (usTicks * 1000 - cycle_cnt) / usTicks;
 }