Explicitly handle realtime kernels

Our original code would not work anymore once the tree merges with regular mainline kernels, as the config option will always exist and thus we'd end up with all kernels being realtime, which is not the expected outcome.

Use the "rt_kernel" (defaults to false) option to mark when the config should be enabled and also add "rt" prefix to the localversion. The localversion is hardcoded in the current realtime trees but again, that will not be the case in the future.

Signed-off-by: Veronika Kabatova vkabatov@redhat.com