Provide a function to change timer interrrupt handlers

Function irq_register_timer_handler(int timer, void (*isr)(void))
lets you change the handler at runtime.  This lets us set handlers
without necessarily knowing the name at link-time.  The old way of
having weakly-linked tmr0_isr functions should still work.

cpu/mc1322x/src/default_lowlevel.h

@@ -30,7 +30,7 @@
  * This file is part of libmc1322x: see http://mc1322x.devl.org
  * for details. 
  *
- * $Id: default_lowlevel.h,v 1.1 2010/06/10 14:49:31 maralvira Exp $
+ * $Id: default_lowlevel.h,v 1.2 2010/11/07 14:27:01 maralvira Exp $
  */
 
 #ifndef LOWLEVEL_H
@@ -42,4 +42,6 @@
 void default_vreg_init(void);
 void uart1_init(uint16_t inc, uint16_t mod, uint8_t samp);
 
+void irq_register_timer_handler(int timer, void (*isr)(void));
+
 #endif