Add SD/MMC Support

[thejpster]

Todo list:

• [x] Enable SPI peripheral
• [x] Write SDMMC protocol driver
• [x] Implement BlockDevice trait
• [x] Implement FAT16/FAT32
• Implement menu commands for:
  • [X] Mounting an SD card (mount)
  • [x] Unmounting an SD card (unmount)
  • [x] Directory Listing for root dir (dir / or just dir)
  • [x] Load file from root dir into RAM (dload filename)
  • [x] Page through text file in root dir (dpage somefile)
  • [x] Hexdump binary file in root dir (ddump somefile)
  • [ ] Directory Listing for arbitrary dir (dir sd0:/foo/bar/baz)
  • [ ] Load file from sub-dir into RAM (dload sd0:/foo/bar/filename)
  • [ ] Page through text file in root dir (dpage sd0:/foo/bar/somefile)
  • [ ] Hexdump binary file in root dir (ddump sd0:/foo/bar/somefile)
  • [ ] Delete file (with wildcards) (del)
  • [ ] Rename / Move file (move)
  • [ ] Make new directory (mkdir)
  • [ ] Delete empty directory (rmdir)
  • [ ] Copy file (copy)

Commands have DOS-like names rather than UNIX like names, because this is more like MS-DOS v3.3 rather than UNIX.

EDIT: I've split this into read-only support, and read-write support.