How to determine the length of the RBF file

Yes, I see, that it can be a problem with NIOS. However, the FPGA image has always trailing 0xFF and the NIOS boot image, as I learned from the bootloader document, is starting != 0xFF. So there should be still a clear start marking.

My application is more basic, the rbf image is written by a standard bootloader to a predefined flash area without length information, but the embedded PS loader should be able to load the correct bit stream length.