[RFC] Define task pipeline

[Claudius Heine <claudius.heine.ext@siemens.com>]

Hi,

I opened an issue for discussion on the github issue tracker[1]. Please 
post comments there.

Here is the content for reference:

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.

Hopefully lines like these
```
addtask ubi_image before do_build after do_copy_boot_files 
do_install_imager_deps do_unpack do_transform_template
```
can be reduced to this
```
addtask ubi_image before do_image after do_rootfs_postprocess
```

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

[1] https://github.com/ilbers/isar/issues/46

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