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/Kconfig b/Kconfig
index 5b55d87..a90d9f9 100644
--- a/Kconfig
+++ b/Kconfig
@@ -7,4 +7,6 @@
 
 comment "Compiler: $(CC_VERSION_TEXT)"
 
+source "scripts/Kconfig.include"
+
 source "arch/$(SRCARCH)/Kconfig"