[top] Implement simple hearbeat scheme with AFC front panel leds

[lerwys]

Problem: It's always nice to know if your firmware was correctly loaded. So we could have a simple heartbeat scheme.

Solution: This could be implmented using the RGB leds available on AFC front panel. Using a simple green LED blinking on 1Hz rate or similar should suffice.

[lerwys]

Done in heartbeat module and implemented in vivado top modules