Thanks RogueM, this information helps me a lot. So the focus is not the boot.txt. I've tried:
1) disconnect the Hifiberry Amp+, the behavior with both HDMI and touchscreen attached works. The touchscreen is working.
2) connecting the Amp+, both HDMI and touchscreen attached, the touchscreen works.
3) connecting the Amp+, only HDMI attached, the HDMI works.
4) Turn everything off, AFTER situation 3, now both HDMI and touchscreen attached, only HDMI works.
My first conclusion: the Amp+ 'feeds' a kind of memory bit inside the Raspberry Pi, telling it to feed only HDMI. Disconnecting all the power has no result. Only disconnecting the Amp+ from the Raspberry Pi, gives a clean status like situation 2.