[U-Boot-Users] How to specify the size of bin?

[甜瓜]

Howdy,
     I know it is a newbie question. Any hint from you are welcome.
In default, u-boot 1.1.5, "make walut_config && make all" will
generate a 256KB bin.
The last 4 bytes of the bin is a jump instruction to the entry point
of u-boot.Is that right?
But my ppc405GP walnut board has 512KB flash for holding u-boot. I wonder how to
build a 512KB bin? Is there any OPTION/SETTING targeting this problem?
Anything else
I should take care?
    Thank you!
---
ShenLei