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>

arch/x86/Makefile

@@ -250,8 +250,8 @@ archclean:
 PHONY += kvmconfig
 kvmconfig:
 	$(if $(wildcard $(objtree)/.config),, $(error You need an existing .config for this target))
-	$(Q)$(CONFIG_SHELL) $(srctree)/scripts/kconfig/merge_config.sh -m -O $(objtree) $(objtree)/.config arch/x86/configs/kvm_guest.config
-	$(Q)yes "" | $(MAKE) oldconfig
+	$(Q)$(CONFIG_SHELL) $(srctree)/scripts/kconfig/merge_config.sh -m -O $(objtree) $(objtree)/.config $(srctree)/arch/x86/configs/kvm_guest.config
+	$(Q)yes "" | $(MAKE) -f $(srctree)/Makefile oldconfig
 
 define archhelp
   echo  '* bzImage      - Compressed kernel image (arch/x86/boot/bzImage)'