I have the RetroPie SD Image installed and did an update (binary images). Even though I looked and looked, I cannot make the RetroArch Master System emulator quit a game with pressing start and select at the same time.
The interesting thing: I can quit games with start+select, as long as I’m using the Atari 2600, MAME, Genesis, NES, Turbografx 16 or SNES emulator. Just for the Sega Master System, it does not seem to work.
this is the interesting part of ~/RetroPie/configs/all/retroarch.cfg :