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 <[e-mail]>
Acked-by: Borislav Petkov <[e-mail]>
Cc: Pekka Enberg <[e-mail]>
Link: http://lkml.kernel.org/r/[e-mail]
Signed-off-by: Ingo Molnar <[e-mail]>

arch/x86/Makefile

···
 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)'