blink_loop.c

#include <avr/io.h>
#include <avr/interrupt.h>
#include <util/delay.h>

int main(int argc, char **argv)
{
    DDRD |= 1<<0;
    PORTD |= 1<<0;
    _delay_ms(500);
    PORTD &= ~(1<<0);
    _delay_ms(500);
    sei();
    // an interrupt left active by the bootloader will be triggered,                                                                                                  
    // which will run __bad_interrupt which resets the program                                                                                                        
    return 0;
}