Stop redefining MOTHERBOARD and #define AT90USB instead. Also put #ifndef AT90USB around the HardwareSerial_h trick, so now Printrboard / Brainwave support compiles on Arduino 1.0.2 as well.

Marlin/stepper.cpp

@@ -478,10 +478,10 @@ ISR(TIMER1_COMPA_vect)
 
     
     for(int8_t i=0; i < step_loops; i++) { // Take multiple steps per interrupt (For high speed moves) 
-      #if MOTHERBOARD != 8 // !teensylu
+      #ifndef AT90USB
       MSerial.checkRx(); // Check for serial chars.
-      #endif 
-      
+      #endif
+
       #ifdef ADVANCE
       counter_e += current_block->steps_e;
       if (counter_e > 0) {