config: revert NO_HZ_FULL=y to NO_HZ_IDLE=y (bnc#826946)
    
    NO_HZ_FULL mode was introduced in v3.10 but still has some
    problems to be worked out. Most notably are issues with
    CPU accounting and how it interacts with power states.
    
    We'll re-enable it when it's been proven to be stable.