block: implement block_read/block_write functions ...

Some drivers use blk->ops->read/write. This bypasses the caching block
layer and was never intended like this. The upper API to the block layer
is the cdev layer. This patch adds block_read and block_write functions
and uses them where appropriate.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>

block: remove unused read_start and read_done ops ...

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>

block: reimplement caching ...

The current caching layer only has a single buffer for
writing and reading. The FAT driver often accesses the
fat and then data again, which currently can't be cached.
Reimplement this with a list of cached chunks. The number
of chunks and their sizes are currently hardcoded, but
that could be easily made configurable.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>

add block support ...

This adds a simple block layer to barebox. Reading and writing
to block devices can be painfully slow without caching, so
add a simple caching layer here.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>