AVR: Fix linking error for AVR platforms that have no ARCH_HAVE_LEDS defined

Minor bug while working on Arduino Mega support. I didn't add support
for ARCH_HAVE_LEDS and have it undefined in .config. To my surprise compilation
failed since AVR code had a related function compiled in.

arch/avr/src/avr/up_createstack.c

@@ -160,7 +160,9 @@ int up_create_stack(FAR struct tcb_s *tcb, size_t stack_size, uint8_t ttype)
       tcb->adj_stack_ptr  = (FAR void *)top_of_stack;
       tcb->adj_stack_size = stack_size;
 
+#if defined(ARCH_HAVE_LEDS)      
       board_autoled_on(LED_STACKCREATED);
+#endif
       return OK;
     }
 

arch/avr/src/common/up_initialize.c

@@ -297,6 +297,8 @@ void up_initialize(void)
 
   up_usbinitialize();
 
+#if defined(ARCH_HAVE_LEDS)     
   board_autoled_on(LED_IRQSENABLED);
+#endif
 }