[COMMANDS] Add a blockdev read command.

This patch adds a read command for block devices, allowing to read data from
it.
Arguments:
- the device name (mandatory)
- the amount of data in bytes to read. If not specified, it will take the
  device default block size.
- the offset in bytes.
[DRIVERS] Temporary MMC hack to do only single block read operations.

As the i.MX uSDHCI driver does not work with multi block operations yet, the
single operation is forced for now.
[DRIVERS] SDHCI pio operations must not be done when using DMA.

It seems that even with DMA activated, the SDHCI driver still does a controller
buffer reading, sometimes leading to a read data buffer overwritting.