Did you have g_mass_storage working yet?
I have it working on my Olimex A20 board following below link, but with some issues.
https://www.olimex.com/forum/index.php?topic=3113.0My issue is:
1) If I connect my device to Ubuntu PC first, then do the "modprobe g_mass_storage...", then all works fine.
2) if I do "modprobe" first, then connect my device to Ubuntu PC, USB Mass Storage does not work.