kconfig: unify rule of config, menuconfig, nconfig, gconfig, xconfig

Unify the similar build rules.

This supports 'make build_config', which builds scripts/kconfig/conf
but does not invoke it.
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>

scripts/kconfig/Makefile

@@ -3,9 +3,6 @@
 # Kernel configuration targets
 # These targets are used from top-level makefile
 
-PHONY += xconfig gconfig menuconfig config localmodconfig localyesconfig \
-	build_menuconfig build_nconfig build_gconfig build_xconfig
-
 ifdef KBUILD_KCONFIG
 Kconfig := $(KBUILD_KCONFIG)
 else
@@ -19,29 +16,24 @@ endif
 # We need this, in case the user has it in its environment
 unexport CONFIG_
 
-xconfig: $(obj)/qconf
-	$(Q)$< $(silent) $(Kconfig)
-
-gconfig: $(obj)/gconf
-	$(Q)$< $(silent) $(Kconfig)
-
-menuconfig: $(obj)/mconf
-	$(Q)$< $(silent) $(Kconfig)
-
-config: $(obj)/conf
-	$(Q)$< $(silent) $(Kconfig)
-
-nconfig: $(obj)/nconf
-	$(Q)$< $(silent) $(Kconfig)
-
-build_menuconfig: $(obj)/mconf
+config-prog	:= conf
+menuconfig-prog	:= mconf
+nconfig-prog	:= nconf
+gconfig-prog	:= gconf
+xconfig-prog	:= qconf
 
-build_nconfig: $(obj)/nconf
+define config_rule
+PHONY += $(1)
+$(1): $(obj)/$($(1)-prog)
+	$(Q)$$< $(silent) $(Kconfig)
 
-build_gconfig: $(obj)/gconf
+PHONY += build_$(1)
+build_$(1): $(obj)/$($(1)-prog)
+endef
 
-build_xconfig: $(obj)/qconf
+$(foreach c, config menuconfig nconfig gconfig xconfig, $(eval $(call config_rule,$(c))))
 
+PHONY += localmodconfig localyesconfig
 localyesconfig localmodconfig: $(obj)/conf
 	$(Q)$(PERL) $(srctree)/$(src)/streamline_config.pl --$@ $(srctree) $(Kconfig) > .tmp.config
 	$(Q)if [ -f .config ]; then 				\