kernel-yocto: set cwd before querying the meta data dir

To aid classifying options and streamling reports, fragments
and classification info are consolidated into a kernel meta
directory.

The name of the meta directory is (somewhat) flexible, and hence
is queried by the tools and created if it doesn't exist.

The recent enhancements to the .config auditing introduced a
new query for the meta directory, but isn't being run in the
kernel's source directory .. and is hence creating an unwanted
file in $TOPDIR. We set the kernel source directory as the cwd
for the call, and everything goes back to where it belongs.

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