the recently introduced files.ppbus.
Also, let the mvme68k port use it's own lpt implementation for now,
so define USE_ALT_LPT option in std.memv68k.
This makes the mvme68k kernels configure and build again.
in preparation for future mvmeppc and mvme88k ports.
This needs a bit if tidying up to make it trully shareable, which will
happen as the new mvme ports are added.