linux: enable PREEMPT_RT

[fhunleth]

This updates the Linux kernel, includes the real-time patch set, and
enables PREEMPT_RT..

[ConnorRigby]

I'm not fully sure how the preempt_rt system works, does this effectively do nothing without configuration giving it the ability to be used, or is there some other tradeoff being made here?

[fhunleth]

@ConnorRigby Yeah, this is a really good question. I was highly skeptical. I was shown some stats today that GPIO interrupt response time in Nerves improves by at least a factor of 10 with the rt patch enabled. The data is pretty convincing. I'm still going to keep this PR unmerged until I get back and can repeat the tests. Given that my assumptions on Linux interrupt latency appear to be clearly wrong, I'm also planning on revisiting Circuits.GPIO decisions. If all this appears to be the case, then Nerves can to be used in situations with much tighter deadlines than I've been telling people and that would be quite interesting.