d/config: Don't build rt or signed kernels

With the changes to the approach used for kernel build configuration with the latest kernels, building of real time flavours and signed kernels snuck back in.

Remove the config for these options as we don't support or use them and in the case of signed kernels, it is breaking or image building.

Signed-off-by: Martyn Welch martyn.welch@collabora.com