Burn this filea into a 32k EPROM and insert it into U36 socket in C128. You can bypass its execution by holding RUN/STOP or C= key during power-up or reset. It will drop you into machine monitor (exit by 'x') or C64 mode.

You still need a bootdisk with at least DeskTop (but existing auto-execs will be executed before that). Boots always to 80 column mode and not much can be done about that. It is somewhat tuned to have centered screen on my TV set so you might have problems with visual. Anyway, after DeskTop is loaded - press RETURN two times and you will switch into 40 column mode.