Define task pipeline

[cmhe]

Currently we don't have a clearly define pipeline of the tasks, especially for image recipes. This makes it difficult to introduce new tasks that should be run at certain points of the build process.

For image recipes I would like to propose the following pipeline:

1. do_rootfs a (virtual) task after which the rootfs directory contains the desired root file system with all packages installed, etc.
2. do_rootfs_postprocess a (virtual) task after which all post processing steps that can not be defined in a package are done.
3. do_image a virtual task after which the images are created in the work directory of the image.
4. do_deploy a (virtual) task after which the output artifacts of the recipe are deployed to the deploy directory.
5. do_build a virtual task that defines the default build process.

This way it should be simpler to order custom and currently implemented task better into the structure.
No more lines like these

addtask ubi_image before do_build after do_copy_boot_files do_install_imager_deps do_unpack do_transform_template

but instead

addtask ubi_image before do_image after do_rootfs_postprocess

I have not looked into optimizing the package build pipeline and other recipe types.