Minor compiler compatibility update for _delay_us().

nuts_bolts.c

@@ -3,7 +3,8 @@
   Part of Grbl
 
   Copyright (c) 2009-2011 Simen Svale Skogsrud
-
+  Copyright (c) 2011-2012 Sungeun K. Jeon  
+  
   Grbl is free software: you can redistribute it and/or modify
   it under the terms of the GNU General Public License as published by
   the Free Software Foundation, either version 3 of the License, or
@@ -43,3 +44,10 @@ void delay_ms(uint16_t ms)
 {
   while ( ms-- ) { _delay_ms(1); }
 }
+
+// Delays variable defined microseconds. Compiler compatibility fix for _delay_us(),
+// which only accepts constants in future compiler releases.
+void delay_us(uint16_t us) 
+{
+  while ( us-- ) { _delay_us(1); }
+}