m68k: use KBUILD_DEFCONFIG

The default defconfig should be one from arch/m68k/configs/

arch/m68k/defconfig was not exactly identical to amiga_defconfig but
also considering how long they have been without any update that doesn't
seem to have been on purpose.

Signed-off-by: Adrian Bunk <adrian.bunk@movial.fi>
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>