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>