(superseded, see OpenWrt git fork)
Here's a rough guide to building OpenWrt SVN for the Bifferboard. This information is subject to change rapidly, and will probably be out of date almost as I write it, but this pertains to OpenWrt trunk as of revision 22979.
These instructions should work OK for 4MB and 8MB devices. You'll need at least Biffboot version 2.7 to get OpenWrt working, because you need the kernelmax value to be in the config block. Please ensure you have a valid config block, and that Biffboot doesn't print any errors on the serial console when it starts. This is needed because OpenWrt looks at the config block (kernelmax) to determine the flash layout.
You can follow the instructions on this page. In particular, pay attention to the bit about extra packages. Basic OpenWrt checkout only gives you the minimal package set.
1) Check out the trunk:
$> svn co -r 22979 svn://svn.openwrt.org/openwrt/trunk
2) Apply the patch at the bottom of the page. It does a couple of things, first is disables the crashlog patch (which doesn't work properly in OpenWrt), and second, it fixes DHCP so it will request the hostname 'biffer'. This means you can just 'telnet biffer' to access your board if you don't have the serial cable.
$> cd trunk
$> patch -p0 < ../bb-r22979.patch
3) Run 'make menuconfig', select the RDC321x target system.
4) Select target profile, "Bifferboard"
*** Note: You may also want to add some extra packages using the menus at this stage ***
5) Run 'make' and wait a while.
Don't flash the image to the board yet! First you have to change Biffboot config.
6) At the Biffboot prompt, ensure the kernelmax value set to 0x0010.
7) At the Biffboot prompt, change the kernel command-line to an empty string. The OpenWrt kernel embeds the necessary parameters in the kernel it generates, and, indeed, any parameters passed from Biffboot may interfere with these.
8) Now flash trunk/bin/rdc/openwrt-rdc-jffs2-64k-bifferboard.img to the board, to get your working OpenWrt system, e.g.:
python bb_eth_upload8.py eth0 00:a1:b2:c3:d4:e5 trunk/bin/rdc/openwrt-rdc-jffs2-64k-bifferboard.img