image: Create separate tasks for rootfs construction

This patch splits the code in lib/oe/image into separate tasks, one per
image type. This removes the need for the simple task graph code and defers
to the bitbake task management code to handle this instead.

This is a good step forward in splitting up the monolithic code and starting
to make it more accessible to people.

It should also make it easier for people to hook in other tasks and processes
into the rootfs code.

Incidentally, the reason this code was all combined originally was due to
limitations of fakeroot where if you exited the session, you lost permissions
data. With pseudo this constraint was removed.

We did start to rework the rootfs/image code previously and got so far with
untangling it however we did prioritise some performance tweaks over splitting
into separate tasks and in hindsight, this was a mistake and should have been done
the other way around. That work was suspended due to changes in the people working
on the project but this split has always been intended, now is the time to finish
it IMO.

There were some side effects of doing this:

* The symlink for the manifest moves to the rootfs-postcommands class and into
  the manifest function.
* There is no seperate "symlink removal" and "symlink creation", they are merged
* The date/time stamps of the manifest and the built images can now be different since
  the tasks can be run separately and the datetime stamp will then be different
  between do_rootfs and the do_image_* tasks.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

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