According to my knowledge , 55001488-02 Rev A Development board is there. Could you please send the
Board number and software ISO image version (Like 40002739_A).
From where you got the IMX51 Reference manual ,you mentioned that "no valid image is found, cpu automaticaly switch to serial downloader" , could you please send the link for that document.
If you think that U-Boot lost , you have to contact nearest digi sales person, may be they can replace the development board.
It looks like U-Boot lost.
Waiting for your reply.