Configure buildroot to automatically build our kernel (and as a side
effect, get the kernel modules into the rootfs image). While we're at
it, defconfig-ify kernel_config.
For now, we reference the tarball at
<http://n900.quitesimple.org/rescueOS/sourcecode/kernel_source.tar.gz>
which is equivalent to commit
a91ce11c777f31ec3d04e60506bcd224f448382e
from the former MeeGo N900 device adaptation project.
