Thanks for your help.

In conclusion :

1.
To change the MAME input configuration:
Start Mame in text mode with typing: ./mame
Lauch a game (rom)
Press ‘Tab’ to enter inside the GUI menu and choose ‘Input (general)’
Change input configuration (keys, joypad…)

2.
To backup and restore Mame input configuration :
-> backup and restore the file : /opt/retropie/emulators/mame4all-pi/cfg/default.cfg
This file is NOT editable.

3.
Other MAME configuration is stored in: /opt/retropie/emulators/mame4all-pi/mame.cfg