BBB: "sd" command

[Jarielle Catbagan]

These series of patches implement the "sd" command with initialization, reading, and writing 
functionality.  The motivation for the implementation of the "sd" command in the BBB port was to
avoid any modification to the core files as of right now.  The SD interface utilizes the SD mode
as opposed to the currently implemented SPI mode SD interface in uMon.  Future plans include 
integrating the SD mode implementation with the core files.