Disable the rt flavours for armhf and arm64 as well

At the moment Apertis does not use the real-time kernels, so let's not waste time on the builders.

This is not meant to land into v2019 before v2019.0 is released, unless we need to take another release anyway.

mentioned in merge request !9 (closed)

mentioned in merge request !8 (merged)

merged