Asking how to install Linux is not an acceptable topic at this forum. As you've admitted, this is the wrong place.
I'm also not sure under what conditions the current Rockbox bootloader will load linux. You will need to manually install the linux file system (instructions for that can be found at the iPodLinux site). Then, if you use the Rockbox tools to convert the kernel .bin file into a .ipod file (I BELIEVE it's the scramble tool, but I'm not sure, you can look at the makefile for the bootloader builds), then all you have to do is click on the kernel.ipod file in Rockbox, and it should find the ext2 partition and be able to boot.