m68k: make multi_defconfig the default defconfig

It seems to match the intention behind multi_defconfig to make it the
default defconfig.

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