kernel-yocto: allow promotion of configuration warnings to errors

kernel-yocto: allow promotion of configuration warnings to errors

It was requested to add the ability to upgrade configuration warnings
to errors, so that they can't be missed in a build.

Add a flag KMETA_AUDIT_WERROR, that when set, triggers a bb.fatal at
the end of configuration checking if any warnings are found.

This is off by default, but is available for those that want to enable
it in their kernel recipe or bbappend.

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