When we send MMC commads that contain data the controller provides no
description of the order of events and we need to be able to send data
from and to the controller "when needed". Changed the code to react
on buffer read and buffer write ready based on interrupts.
Change-Id: I60c9140bf0e45b74be6475054564d4e1bd89f21e
* Add dummy driver to allow independent testing of mmcblk.
* Always build the mmc driver to prevent breakage.
* Allow to specify the mmc driver to be used at load time.
Change-Id: I4e14b912fb8f3612e252864b53733968b23ac023
