rvl-stsd driver added

We have added support for using SD cards inserted into the front slot of the Nintendo Wii.

The rvl-stsd platform driver allows one to use SD cards without the need of extra hardware. Support for both reading and writing is available. The IO performance is also _much_ better than the classic SD card adapter driver (gcn-sd).

Now it is practical and possible to use filesystems from SD cards :)

The source code changes are already available through the project's cvs service at http://gc-linux.cvs.sourceforge.net/gc-linux/linux .