Wait wait wait! I just re-read your first post.

You aren't doing it right... "download.c" is just for flash kernel and flash logo writes. The only files you're supposed to write with "download.c" are .ZIMAGE files and boot logo files.

You need the full upgrader program to install an .upgrade file.

Follow the instructions here carefully. I don't think Linux is an option, I think you need a Windows box. It *might* work under WINE.