x86, platform, kconfig: move kvmconfig functionality to a helper

The new mergeconfig helper makes it easier to add other partial
configurations similar to kvmconfig.  Architecture-independent portions
of those partial configurations should go in
kernel/configs/${name}.config, and architecture-dependent portions
should go in arch/${arch}/configs/${name}.config.

Based on a patch by Luis R. Rodriguez <mcgrof@suse.com>.
Originally-Signed-off-by: Luis R. Rodriguez <mcgrof@suse.com>

Modified to make the helper name more general than just virtualization,
support architecture-dependent and architecture-independent partial
configurations, move the helper and kvmconfig to
scripts/kconfig/Makefile, and factor out more of the common file path.

Signed-off-by: Josh Triplett <josh@joshtriplett.org>

arch/x86/Makefile

@@ -247,12 +247,6 @@ archclean:
 	$(Q)$(MAKE) $(clean)=$(boot)
 	$(Q)$(MAKE) $(clean)=arch/x86/tools
 
-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 $(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)'
   echo  '  install      - Install kernel using'
@@ -266,5 +260,4 @@ define archhelp
   echo  '                  bzdisk/fdimage*/isoimage also accept:'
   echo  '                  FDARGS="..."  arguments for the booted kernel'
   echo  '                  FDINITRD=file initrd for the booted kernel'
-  echo  '  kvmconfig	- Enable additional options for guest kernel support'
 endef

scripts/kconfig/Makefile

@@ -104,6 +104,19 @@ endif
 %_defconfig: $(obj)/conf
 	$(Q)$< --defconfig=arch/$(SRCARCH)/configs/$@ $(Kconfig)
 
+configfiles=$(wildcard $(srctree)/kernel/configs/$(1).config $(srctree)/arch/$(SRCARCH)/configs/$(1).config)
+
+define mergeconfig
+$(if $(wildcard $(objtree)/.config),, $(error You need an existing .config for this target))
+$(if $(call configfiles,$(1)),, $(error No configuration exists for this target on this architecture))
+$(Q)$(CONFIG_SHELL) $(srctree)/scripts/kconfig/merge_config.sh -m -O $(objtree) $(objtree)/.config $(call configfiles,$(1))
+$(Q)yes "" | $(MAKE) -f $(srctree)/Makefile oldconfig
+endef
+
+PHONY += kvmconfig
+kvmconfig:
+	$(call mergeconfig,kvm_guest)
+
 # Help text used by make help
 help:
 	@echo  '  config	  - Update current config utilising a line-oriented program'
@@ -124,6 +137,7 @@ help:
 	@echo  '  randconfig	  - New config with random answer to all options'
 	@echo  '  listnewconfig   - List new options'
 	@echo  '  olddefconfig	  - Same as silentoldconfig but sets new symbols to their default value'
+	@echo  '  kvmconfig	  - Enable additional options for guest kernel support'
 
 # lxdialog stuff
 check-lxdialog  := $(srctree)/$(src)/lxdialog/check-lxdialog.sh