If you're using custom hardware, or want to replace the ROM monitor on your board, you'll need a boot loader that knows how to decompress and transfer control to the kernel. The current trend is to include this functionality in the ROM monitor, as is the case with ppcboot.
Some of this is board specific, but you can assemble the bits from what's already available on the net: http://lists.linuxppc.org/listarcs/linuxppc-embedded/199911/msg00034.html
For PreP and PowerPlus SBC's, see: http://members.home.net/mmporter/linux/ http://members.home.net/mmporter/linux/linuxppc/preploader.tgz http://members.home.net/mmporter/linux/linuxppc/bugboot-0.3.tar.gz
The old bootloader included in the kernel source tree, originally written for the Motorola MBX board, and since ported to a wide range of platforms. This is the start-up component of the zImage that knows how to decompress the kernel. It is now being rendered obsolete by ppcboot.