sched/rt, Kconfig: Introduce CONFIG_PREEMPT_RT

Add a new entry to the preemption menu which enables the real-time support
for the kernel. The choice is only enabled when an architecture supports
it.

It selects PREEMPT as the RT features depend on it. To achieve that the
existing PREEMPT choice is renamed to PREEMPT_LL which select PREEMPT as
well.

No functional change.

Signed-off-by: Thomas Gleixner <[email protected]>
Acked-by: Paul E. McKenney <[email protected]>
Acked-by: Steven Rostedt (VMware) <[email protected]>
Acked-by: Clark Williams <[email protected]>
Acked-by: Daniel Bristot de Oliveira <[email protected]>
Acked-by: Frederic Weisbecker <[email protected]>
Acked-by: Peter Zijlstra (Intel) <[email protected]>
Acked-by: Marc Zyngier <[email protected]>
Acked-by: Daniel Wagner <[email protected]>
Acked-by: Luis Claudio R. Goncalves <[email protected]>
Acked-by: Julia Cartwright <[email protected]>
Acked-by: Tom Zanussi <[email protected]>
Acked-by: Gratian Crisan <[email protected]>
Acked-by: Sebastian Siewior <[email protected]>
Cc: Andrew Morton <[email protected]>
Cc: Christoph Hellwig <[email protected]>
Cc: Greg Kroah-Hartman <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: Lukas Bulwahn <[email protected]>
Cc: Mike Galbraith <[email protected]>
Cc: Tejun Heo <[email protected]>
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Ingo Molnar <[email protected]>

arch/Kconfig

@@ -801,6 +801,9 @@ config ARCH_NO_COHERENT_DMA_MMAP
 config ARCH_NO_PREEMPT
 	bool
 
+config ARCH_SUPPORTS_RT
+	bool
+
 config CPU_NO_EFFICIENT_FFS
 	def_bool n