LPC1857 processor has a factory-supplied boot loader that can take ISP (In-System programming) commands via UART0 port(if we call it to update the flash. We'd like to to use USB rather than UART0 to update flash. Does it called "secondary USB bootloader"? Can we use MCB1857U board to implemented the function? If yes, I'd like to have some information about it, such as application note and user's guide.
Not sure why you put "USB secondary ISP bootloder for LPC1857" inside quotes.
If that is a product you have already found somewhere on NXP sites, then it really is quite likely that the LPC1857 will support it.
No - I have not looked if NXP have a secondary boot loader specifically targeted for your processor or if you will have to spend own time trying to port some of their code. Quite a number of NXP chips have very similar peripherial functionality, even if the processor core and amount of peripherials, memory etc differs.
But the only route you are going to get updates using USB is if - any of NXP code is directly usable. - you port NXP secondary boot loader to your target processor. - you add USB support in your main application and have it download a new application to a secondary storage, and have a small secondary boot loader that either switches memory area or copies the new application binary into the "normal" address space.