Here's what I did to build my own kernel and get Gentoo loaded:

Kernel Setup:
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Get u-boot sources: http://sourceforge.net/projects/u-boot
make walnut_config && make then put tools/mkimage in your path
Copy the default walnut config to .config, configure and then make uImage
emerge tftp-hpa
Setup /etc/conf.d/in.tftpd (default tftp dir, any other settings)
Copy arch/ppc/boot/images/uImage to your tftp dir

RootFS Setup:
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Get the 2005.0 uclibc embedded stage3 from the Gentoo mirrors
Since I'm using a ppc machine, I chroot into the stage, unmerge devfs and
emerge udev
Copy in /lib/modules/2.6.15 (for modules.dep on boot)
Edit /etc/fstab to reflect your nfs setup
Edit /etc/inittab for serial console
Any other customizations required (ethernet, etc.)
Set a root password

Serial console to board (minicom works fine, set to 9600,N,1 no hardware
flow control):
setenv ethaddr AA:BB:CC:DD:EE:FF
setenv ipaddr <ipaddr>
setenv serverip <tftp serverip>
setenv bootargs -s console=ttyS0,9600 root=/dev/nfs rw nfsroot=<nfs
serverip>:/root ip=<ipaddr>::<gateway>:<netmask>:<hostname>
tftpboot 400000 uImage
bootm 400000