Update for binding _delay_us to delayMicroseconds

AVR compatibility

teensy3/util/delay.h

@@ -1,3 +1,7 @@
+#ifndef _delay_us
+#define _delay_us(n) delayMicroseconds(n)
+#endif
+
 #ifndef _delay_ms
 #define _delay_ms(n) delay(n)
 #endif