classes/populate_sdk_ext: hide build configuration in devtool build* output

classes/populate_sdk_ext: hide build configuration in devtool build* output

The configuration of the build system within the extensible SDK is
fixed, so there's not a lot of point in showing it; plus it just gets in
the way of the output that's interesting to the user in this context. So
let's hide it within the extensible SDK.

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>