Reported by Nikolaus Schaller, Jan 16, 2011

there should be some init to do this:

insmod g_file_storage.ko file=/dev/mmcblk0p1 stall=0 removable=1 

The question may be which memory device we want to share (SD card, 
NAND Flash). Sharing NAND may not work with file_storage since it 
provides a raw sector interface.