kconfig: add basic helper macros to scripts/Kconfig.include

Kconfig got text processing tools like we see in Make.  Add Kconfig
helper macros to scripts/Kconfig.include like we collect Makefile
macros in scripts/Kbuild.include.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Ulf Magnusson <ulfalizer@gmail.com>
diff --git a/arch/x86/um/Kconfig b/arch/x86/um/Kconfig
index a992f8e..9d529f2 100644
--- a/arch/x86/um/Kconfig
+++ b/arch/x86/um/Kconfig
@@ -3,6 +3,8 @@
 
 comment "Compiler: $(CC_VERSION_TEXT)"
 
+source "scripts/Kconfig.include"
+
 source "arch/um/Kconfig.common"
 
 menu "UML-specific options"