m68k: merge the mmu and non-mmu kernel/Makefiles

The few differences between the mmu and non-mmu kernel/Makefiles can
easily be handled inside of a single Makefile. Merge the 2 back into
a single Makefile.

Signed-off-by: Greg Ungerer <gerg@uclinux.org>
Acked-by: Sam Ravnborg <sam@ravnborg.org>