remove more kernels modules
update tmpfs mountpoint
add more packages to the build environment
move hook scripts to a hooks directory

This cleans up the Dockerfile and move all hook scripts to a dedicated
directory.
exclude kernel and initrd from squashfs image

There's no need to have the kernel and the initrd in the squashfs
rootfs.
clean up the chroot

This deletes some larger binaries and other files from the chroot
environment.
readme: update description & add instructions
Merge pull request #7 from unclejack/shrink_image

Shrink image & rearrange scripts