m68knommu: create config options for CPU classes

There are 3 families of CPU core types that we support in the m68knommu
architecture branch. They are

. traditional 68000
. CPU32 (a 68020 core derivative without MMU or bitfield instructions)
. ColdFire

It will be useful going forward to have a CONFIG_ option defined for
each type. We already have one for ColdFire (CONFIG_COLDFIRE), so add
for the other 2 families, CONFIG_M68000 and CONFIG_MCPU32.

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