block: add simple i/o scheduler

The Simple I/O scheduler is an extremely simple scheduler,
based on noop and deadline, that relies on deadlines to
ensure fairness. The algorithm does not do any sorting but
basic merging, trying to keep a minimum overhead. It is aimed
mainly for aleatory access devices (eg: flash devices).

Signed-off-by: Humberto Borba <humberos@gmail.com>
config: add simple i/o as default scheduler

Signed-off-by: Humberto Borba <humberos@gmail.com>