Dear Giuseppe Modugno,
I think the best solution is to use Linux on the source board to copy the good U-Boot environment in a image file. On the destination board, I can download (by tftp) the image and store it on the Flash at the correct address. In this way, I'll use only U-Boot on the destination board to retrieve the good environment. From that, I can install kernel and root filesystem.
No, that is not a good idea, as this way you would also copy board-specific settings lime serial number, MAC address etc.
I also have such a requirement. So what I am doing is that the variables that is common to all boards are kept as a script which my terminal program (used to access serial port) can run. So in the first boot, all these variables plus the board specific things like serial number, MAC address, etc. will be set and saved to flash.
Regards, Jayakrishnan