It was noticed that during the boot of the AVR the Z80 was kept in wait state. Then an Enable signal was added simply by changing the Data input of the flip flop (pin 12) from fixed "1" to a pin coming from AVR. This pin shall be pulled down, so during AVR boot the Enable signal stays in "0" and no wait state will be generated.

When the AVR is ready to work it can push a "1" on the Enable signal to enable the wait state generation.