I have custom BF537 board with 8MB flash on it. Blackfin uses one GPIO line as adr21 to flash to access "upper" 4MB. I made a small modifications to uboot that let access all 8MB (it was already presented here in another thread). Now I faced a problem with bootm command that can not boot images from "upper" 4MB of flash. Here is the patch to fix that.
Be careful however, that all image has to be in one "bank" because GPIO address lines are set only once. Then kernel itself has to set these lines right. Anyway now after a few tests it seems the the patch is working :-).