Previous error message for invalid kunitconfig was vague. Added to it so
that it lists invalid fields and prompts for them to be removed.  Added
validate_config function returning whether or not this kconfig is valid.

Signed-off-by: Heidi Fahim <heidifahim@google.com>
Reviewed-by: Brendan Higgins <brendanhiggins@google.com>
Tested-by: Brendan Higgins <brendanhiggins@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>