x86/platform: Fix "make O=dir kvmconfig"

x86/platform: Fix "make O=dir kvmconfig"

Running:

	make O=dir x86_64_defconfig
	make O=dir kvmconfig

the second command dirties the source tree with file ".config",
symlink "source" and objects in folder "scripts".

Fixed by using properly prefixed paths in the arch Makefile.

Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Acked-by: Borislav Petkov <bp@suse.de>
Cc: Pekka Enberg <penberg@kernel.org>
Link: http://lkml.kernel.org/r/1397377568-8375-1-git-send-email-borneo.antonio@gmail.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>