kernel-yocto: make # is not set matching more precise

Integrating the following commit:

    symbol_why: tighten 'is not set' matching

    Commented lines that contained CONFIG_FOO ... were being picked up
    as option lines and reported as invalid.

    We make the regex more explicit and only match on spaces before
    CONFIG_ to declare if a 'is not set' line is invalid.

Signed-off-by: Bruce Ashfield <bruce.ashfield@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>