Kconfig: Convert CMD_READ to Kconfig

Convert this option and enable it in sandbox. Also correct a bug which
was introduced with the block-device driver model conversion.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
6 files changed