Shrink image & rearrange scripts

This PR does a few things:
- removes more unneeded kernel modules from the rootfs
- updates the tmpfs mountpoint to stop the init scripts from complaining
- adds squashfs-tools to the build environment
- moves hook scripts to a dedicated hooks directory to make the Dockerfile shorter
- remove some large binaries from the chroot environment
- updates the readme to add a project description

/cc @tianon @SvenDowideit

The resulting ISO is now about 55 MB.