kconfig: add 'filename' and 'lineno' built-in variables

The special variables, $(filename) and $(lineno), are expanded to a
file name and its line number being parsed, respectively.

Suggested-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Kees Cook <keescook@chromium.org>