block: extend generic driver API (Ref #750)

* allow to handle a maximum of packets in parallel
  that fits free slots in the ack queue
* stop processing packets, when the driver can't handle
  more requests in parallel, and resume packet handling,
  when the driver is ready again

	os/src/drivers/sd_card/exynos5/driver.h
part_blk: re-design to work event driven

Fix #750