Add CONFIG_PREEMPT_NONE

Gilles's recommendation:
http://permalink.gmane.org/gmane.linux.real-time.xenomai.users/18499

    No, as I said, in my experience, turning CONFIG_PREEMPT off,
    setting it to CONFIG_PREEMPT_NONE, results in shorter worst case
    latencies for Xenomai.

Fixes https://github.com/zultron/kernel-rt-deb/issues/2