m68knommu: fix 68000 CPU link with no platform selected

If building for a nommu m68k classic CPU and no platform (board) is
selected then the final link fails with:

     LD      vmlinux.o
   m68k-linux-ld: cannot find arch/m68k/kernel/head.o: No such file or directory
   make: *** [Makefile:1158: vmlinux] Error 1

Not selecting a platform is ok, that is a generic 68000 system build.
All of the platform selections are for 68328 variants.

The underlying problem is that the CPU config option (CONFIG_M68000)
ends up not being set, it is currently only selected by one of the
platform choices.

Change CONFIG_M68000 so that it is always enabled for the nommu m68k
classic configuration.

Signed-off-by: Greg Ungerer <gerg@linux-m68k.org>