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: NAdrian Bunk <adrian.bunk@movial.fi>
Signed-off-by: NGeert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>