add delay ms safe & add led heart beat

Inc/led.h

@@ -8,5 +8,6 @@ void led_init(void);
 void led_on(void);
 void led_off(void);
 void led_toggle(void);
+void led_heart_beat(void);
 
 #endif // LED_H

Inc/systick.h

@@ -24,4 +24,7 @@ void delay_ms(uint32_t count);
 /* delay a time in microseconds */
 void delay_us(uint32_t count);
 
+/* delay function that doesn't interfere with SysTick interrupt */
+void delay_ms_safe(uint32_t count);
+
 #endif /* SYS_TICK_H */