Grub on the OLPC

[John Mitchell]

Hello, I just had a small question, I've successfully been able to get
my xo-laptop to boot grub using an .elf file. But my question is, how do
I configure the .elf to mount my USB flash drive? by default it
automatically detects the SD card slot, even if there is no SD card
inside. I did manage to insert an SD card, but grub kept complaining
that it was an Unkown File System. 

Could someone please point me in the right direction? Also, if anyone
knows what kind of things an .elf can contain for the XO to execute it
please let me know, I'm interested in seeing if I can make a NES
emulator.