On the following link you can find all the firmware images that are available for the Minnowboard:
However we have consulted with our engineers and they confirmed to us that Atom E38XX series processor doesn't support the Simple Firmware Interface and that this feature is not on Plan of Record.
We'll always try to answer your questions on a timely manner and on the best way that we can. However I would like to suggest the following support channels for the Minnowboard:
Cirtcuitco (the manufacture of the board), has access to more information regarding the design of the board, and can provide you with better support, also on their forums you will find experienced Minnowboard users.