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>