m68k/Kconfig: Separate classic m68k and coldfire early

While you can build multiplatform kernels for machines with classic
m68k processors, you cannot mix support for classic m68k and coldfire
processors. To avoid such hybrid kernels, introduce CONFIG_M68KCLASSIC
as an antipole for CONFIG_COLDFIRE, and make all specific processor
support depend on one of them.
All classic m68k machine support also needs to depend on this.

The defaults (CONFIG_M68KCLASSIC if MMU, CONFIG_COLDFIRE if !MMU) are
chosen such to make most of the existing configs build and work.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Greg Ungerer <gerg@uclinux.org>