Ok, here's a quick summary to get it working, but you should note that we are dealing with the development branch (trunk) and things change all the time, so things may break.
What I did....
1. Check out the code: svn co http://svn.openwrt.org/openwrt/trunk trunk
2. cd trunk
3. Update the feeds: ./scripts/feeds update
4. Install the feeds: ./scripts/feeds install
4b. make package/symlinks
5. Configure: make menuconfig
- Target System: Lantiq GPON/XWAY
- Subtarget: AR9
- Target Profile: WBMR - Buffalo WMBR-HP-G300H
- Target Images: I use squashfs, I haven't tested jffs2
- You might want to enable luci (Luci/Collections/Luci)
- Enable wireless: Kernel Modules/Wireless Drivers/kmod-ath9k
- Enable hostapd: Network/hostapd and Network/hostapd-utils
6. Save the config
Note: when you select packages make sure they go to "*" and not "M", that way they will be built into the image and you won't have to install them separately.
Then in bin/lantiq you will find "openwrt-lantiq-ar9-WBMR-squashfs.image"
Copy this to the router (I usually put it in /tmp), then you can login (assuming you already have dd-wrt on it) and change to /tmp and flash it with: mtd -r write openwrt-lantiq-ar9-WBMR-squashfs.image linux
Please note: it takes a fair amount of time to boot after this as it creates all the filesystem stuff on the remaining flash ... give it 5 - 10 mins before getting worried.
DISCLAIMER: it worked for me, but I cannot be responsible if you brick your router ... I did a couple of times and had to recover by opening it up and attaching a serial console.