sandbox: avoid duplicate backslash input

When using SDL for input the SDL key codes are first converted to Linux key
codes and then to matrix entries of the cross wired keyboard.

We must not map any key code to two different places on the keyboard. So
comment out one backslash position.

Update the rest of the file from Linux 5.7.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>