Thanks for the clarification, Tormod - and for the code, of
course! 
I’ve downloaded also the ST manual for DFUse, and will
look into it harder - at first sight it seemed to me that
the DFUse protocol does contain a few extensions, but
is mostly the same stuff as standard DFU.
http://www.st.com/stonline/products/literature/an/17068.pdf
Antonio