Supporting block I/O contexts

UBIFS sits on top of the UBI, which sits on top of raw NAND flash, while the hardware industry is now moving towards block based storage such as eMMC. While it would be possible to do UBI on top of an eMMC to get some of the performance back, in the discussion we had between Linaro and some of the eMMC vendors, we ended up discarding that idea. Instead, focusing on improving performance on ext4 and btrfs on flash based block devices is something we are planning to spend more time on together.