README: distro kernel should just work
qemu: don't tell how much memory use

Use the default from qemu and let the user override that from the
command-line.
initrd: modprobe cannot find something that cannot be compiled as module

For some reason, AUFS from the git tree cannot be compiled as a
module. Therefore, it is not included in `modules.builtin` and modprobe
cannot know anything about it.
modules: some modules need to be builtin
modules: we need initrd support as well