image/rootfs-postcommands: Separate out post rootfs commands to separate class
Reading image.bbclass is a little difficult as it has many post rootfs
helper functions and its hard to separate those from the core contents
of the rootfs/image code.
Moving it to a separate class would be one way of making it clearer
what these functions are. There are some comment layout improvements
but no code changes.
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
