Okay, my first try with the addition of SPI-Flash did not work, which may have many different reasons:     1. The wrong chip. I only had 64mb-chips at hand and ordered the version, that chwe mentioned (winbond 25q128jvsq). 2. Defective chip, bad soldering (did resolder twice)? 3. The need to add or remove a resistor somewhere? 4. A complete misconception on my side, how the things are working on this platform. I did not find any documentation regarding this to