[Linaro-validation] board state logging

I had a request today to produce some information about how long each board spends running jobs, idling, or offline as percentages for the past month, quarter, etc. This will be even more useful as we start doing things like offlining boards automatically when there is a problem. https://blueprints.launchpad.net/lava-scheduler/+spec/lava-board-state-loggi...

I know it won't be feasible to get historical data for offlinined boards, but I'd like if we could make it possible from this point forward. So this might be a good thing to try to get done during the hacking sessions this week.

Thanks, Paul Larson