Override config for MAME libretro keyboard mapping in RetroArch

MAME.cfg

## Keyboard input override for MAME-libretro
## place this in $rgui_config_directory/MAME/MAME.cfg (for the Git version of MAME) to have it loaded automatically when override loading is enabled
## Also applicable to any core that makes use of RetroKeyboard extensively.

# Remove all RetroPad keyboard mappings to avoid conflicts with MAME keyboard mappings
# Note: you will need a joypad or mouse to navigate RetroArch's menu with these disabled.
input_player1_a = "nul"
input_player1_b = "nul"
input_player1_y = "nul"
input_player1_x = "nul"
input_player1_select = "nul"
input_player1_start = "nul"
input_player1_l = "nul"
input_player1_r = "nul"
input_player1_l2 = "nul"
input_player1_r2 = "nul"
input_player1_l3 = "nul"
input_player1_r3 = "nul"
input_player1_left = "nul"
input_player1_right = "nul"
input_player1_up = "nul"
input_player1_down = "nul"

# Block RetroArch hotkeys unless this key is held to avoid conflicts with MAME keyboard mappings
input_enable_hotkey = "scroll_lock"

# If using hotkeys on a joypad (e.g. Xinput Guide button to open RetroArch menu) then map one of these to enable that hotkey button.
# input_enable_hotkey_btn = "10" # for Xinput Guide button
input_enable_hotkey_btn = "nul"
input_enable_hotkey_axis = "nul"