x86: move defconfig files for i386 and x86_64 to x86

With some small changes to kconfig makefile we can now
locate the defconfig files for i386 and x86_64 in
the configs/ subdirectory under x86.
make ARCH=i386 defconfig and make defconfig
works as expected also after this change.
But arch maintainers shall now update a defconfig file in
the configs/ directory.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
diff --git a/arch/x86/Makefile b/arch/x86/Makefile
index 65077d7..3095973 100644
--- a/arch/x86/Makefile
+++ b/arch/x86/Makefile
@@ -1,6 +1,9 @@
 # Unified Makefile for i386 and x86_64
 
-# No need to remake these files
+# select defconfig based on actual architecture
+KBUILD_DEFCONFIG := $(ARCH)_defconfig
+
+# # No need to remake these files
 $(srctree)/arch/x86/Makefile%: ;
 
 ifeq ($(ARCH),i386)