"map -r" doesn't list anything like fs0: fs1: or fs2: but only blk0 through 7. When looking at any of these with the dir command, they all appear empty.
UPDATE: While writing this it struck me that it might have to do with the filesystem on the USB-drive. I was NTFS, so I tried to reformat it as FAT32 and not it is recognised as blk0 in the EFI-prompt.
Please add this as one of the first things in your guides and read-me files.