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>

arch/m68k/Makefile

@@ -13,6 +13,8 @@
 # Copyright (C) 1994 by Hamish Macdonald
 #
 
+KBUILD_DEFCONFIG := amiga_defconfig
+
 # override top level makefile
 AS += -m68020
 LDFLAGS := -m m68kelf