m68knommu: make ColdFire SoC selection a choice

It would be nice if we could support multiple ColdFire SoC types in a
single binary - but currently the code simply does not support it.
Change the SoC selection config options to be a choice instead of
individual selectable entries.

This fixes problems with building allnoconfig, and means that a sane
linux kernel is generated for a single ColdFire SoC type.

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