How do I calculate the checksum for rockbox.ipod file?

[atarax]

I am working on a script to change the image of the bootsplash, like Logo Swapper, for Linux. I managed to change the image data in the rockbox.ipod file, but I get a checksum error on boot. So I need to change these first four bytes of the rockbox.ipod file, too. But I have no clue, how to calculate this value. I would really appreciate any replies! Thank you.
Post Merge: August 05, 2011, 12:51:57 AMReported

[nls]

The build system uses the scramble tool to add a header with checksum to the rockbox.bin file. I'd suggest either editing the bin and generating the .ipod file with scramble or just changing the image in the source and rebuilding.