I just tried adding the button mapping below to the retroarch.cfg located in /opt/retropie/configs/nes bit this did absolutely nothing. The NES-buttons A and B are still mapped to X and SQUARE for some unknown reason.

input_device = “Sony PLAYSTATION(R)3 Controller”
input_driver = “udev”
input_a_btn = “14”
input_b_btn = “15”

14 is the SQUARE and 15 is X on the PS3-controller.

I cant figure out which cfg-file tells NES games these strange A and B-button mappings.