sdmmc test: fix issue running on Linux systems
sdmmc test: fix issue running on Linux systems

dd works differently on Mac vs. Linux.  Detect the system and use the
appropriate command line parameters.

jbush001/NyuziProcessor,hoangt/NyuziProcessor,jbush001/NyuziProcessor,jbush001/NyuziProcessor,FulcronZ/NyuziProcessor,hoangt/NyuziProcessor,jbush001/NyuziProcessor,FulcronZ/NyuziProcessor,hoangt/NyuziProcessor,FulcronZ/NyuziProcessor,FulcronZ/NyuziProcessor,hoangt/NyuziProcessor,jbush001/NyuziProcessor,jbush001/NyuziProcessor