kconfig: run olddefconfig instead of oldconfig after merging fragments

'make olddefconfig' is non-interactive, so we can drop 'yes'.
The behavior is equivalent.
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>

scripts/kconfig/Makefile

@@ -94,7 +94,7 @@ configfiles=$(wildcard $(srctree)/kernel/configs/$@ $(srctree)/arch/$(SRCARCH)/c
 %.config: $(obj)/conf
 	$(if $(call configfiles),, $(error No configuration exists for this target on this architecture))
 	$(Q)$(CONFIG_SHELL) $(srctree)/scripts/kconfig/merge_config.sh -m .config $(configfiles)
-	+$(Q)yes "" | $(MAKE) -f $(srctree)/Makefile oldconfig
+	$(Q)$(MAKE) -f $(srctree)/Makefile olddefconfig
 
 PHONY += kvmconfig
 kvmconfig: kvm_guest.config