after many set backs (32bit arm-gcc on 64bit system, ISE under Linux), I managed to build a FSBL with ISE 14.7 which boots. Fortunately I can try and error with FSBLs because I have a JTAG connection (don't try this at home, if you cannot connect JTAG to reflash). But I did not yet patch the FSBL, because I ran out of time again. I will try and report, if I find time again, sorry....
Joerg