um: stop abusing KBUILD_KCONFIG

Instead create a arch/um/Kconfig file that just includes the actual
per-arch Kconfig file.  Note that we use HEADER_ARCH to find the
per-arch Kconfig file as that variable already includes the
normalization from i386 or x86_64 to x86.
Signed-off-by: Christoph Hellwig <hch@lst.de>
Acked-by: Richard Weinberger <richard@nod.at>
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>

arch/um/Kconfig

+# SPDX-License-Identifier: GPL-2.0
+
+source "arch/$(HEADER_ARCH)/um/Kconfig"

arch/um/Makefile

@@ -113,8 +113,6 @@ define archhelp
   echo '		   find in the kernel root.'
 endef
 
-KBUILD_KCONFIG := $(HOST_DIR)/um/Kconfig
-
 archheaders:
 	$(Q)$(MAKE) -f $(srctree)/scripts/Makefile.asm-generic \
 	            kbuild-file=$(HOST_DIR)/include/asm/Kbuild \
@@ -176,4 +174,4 @@ endef
 include/generated/user_constants.h: $(HOST_DIR)/um/user-offsets.s
 	$(call filechk,gen-asm-offsets)
 
-export SUBARCH USER_CFLAGS CFLAGS_NO_HARDENING OS DEV_NULL_PATH
+export HEADER_ARCH SUBARCH USER_CFLAGS CFLAGS_NO_HARDENING OS DEV_NULL_PATH

arch/x86/um/Kconfig

 # SPDX-License-Identifier: GPL-2.0
-mainmenu "User Mode Linux/$(SUBARCH) $(KERNELVERSION) Kernel Configuration"
-
-comment "Compiler: $(CC_VERSION_TEXT)"
-
-source "scripts/Kconfig.include"
 
 source "arch/um/Kconfig.common"