Add a led blinking code and a build script

src/main.c

@@ -0,0 +1,18 @@
+#include <avr/io.h>
+#include <util/delay.h>
+
+#define BLINK_DELAY_MS  1000
+
+int main (void)
+{
+    /*  set pin 25 of PORTB for output*/
+    DDRA |= _BV(DDA3);
+    while(1) {
+        /*  set pin 25 high to turn led on  */
+        PORTA |= _BV(PORTA3);
+        _delay_ms(BLINK_DELAY_MS);
+        /*  set pin 25 low to turn led off */
+        PORTA   &=  ~_BV(PORTA3);
+        _delay_ms(BLINK_DELAY_MS);
+    }
+}