preempt_rt: Add a warning regarding cpu schedulers availability when preempt_rt is enabled

Fixes https://github.com/Frogging-Family/linux-tkg/issues/635

linux-tkg-config/prepare

@@ -285,6 +285,7 @@ _set_cpu_scheduler() {
   fi
 
   if [ "${_preempt_rt}" = "1" ]; then
+    warning "! Since you have enabled _preempt_rt, incompatible cpu schedulers will not be available !"
     if [[ "${_avail_cpu_scheds[*]}" =~ "bore" ]]; then
       _avail_cpu_scheds=("cfs" "bore")
     else