m68k/m68knommu: merge MMU and non-MMU string.h

The MMU and non-MMU string.h varients (string_no.h and string_mm.h)
and almost the same. Switch to using the string_mm.h one, merging
in the necessary ColdFire support.

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