rcutorture: Add SRCU Kconfig-fragment files

Use .boot facility to ease inclusion of SRCU into automated testing.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Greg KH <gregkh@linuxfoundation.org>

tools/testing/selftests/rcutorture/configs/SRCU-N

+CONFIG_RCU_TRACE=n
+CONFIG_SMP=y
+CONFIG_NR_CPUS=8
+CONFIG_HOTPLUG_CPU=y
+CONFIG_PREEMPT_NONE=y
+CONFIG_PREEMPT_VOLUNTARY=n
+CONFIG_PREEMPT=n
+CONFIG_PRINTK_TIME=y

tools/testing/selftests/rcutorture/configs/SRCU-N.boot

+rcutorture.torture_type=srcu

tools/testing/selftests/rcutorture/configs/SRCU-P

+CONFIG_RCU_TRACE=n
+CONFIG_SMP=y
+CONFIG_NR_CPUS=8
+CONFIG_HOTPLUG_CPU=y
+CONFIG_PREEMPT_NONE=n
+CONFIG_PREEMPT_VOLUNTARY=n
+CONFIG_PREEMPT=y
+CONFIG_PRINTK_TIME=y

tools/testing/selftests/rcutorture/configs/SRCU-P.boot

+rcutorture.torture_type=srcu