The config.seed files are included as far as I remember. This should give you a good start what to include in your own “tiny” build. All the 32/4 warnings still apply but you should be able to make a working firmware like in the thread I mentioned.
Copy this file to your checked out openwrt source, eg.: cp /path/to/config.seed /path/to/openwrt-src/.config.
Then do make defconfig && make menuconfig, remove unneded devices and add your device.
Then simply build with make download world -j4
Before you do make defconfig you have to fetch all feeds with ./scripts/feeds update -a && ./scripts/feeds install -a from inside openwrt source dir.
Kubuntu is like Ubuntu with KDE instead of Gnome as a desktop environment. I use the scripts and instructions from hnyman to set up the build environment detailed in this thread. You may have to make some changes in the configuration to build for your router.