x86: switch image generation to new code

This commit introduces few related changes which need to be done in
single commit to keep images buildable between git revisions. In result
it retains all previous image creation possibilities with slight name
change of generated images. Brief summary of the commit:

* Split up image generation recipe to smaller chunks to make it more
  generic and reusable.

* Make iso images x86 specific and drop their definition as root
  filesystem.

* Convert image creation process to generic code specified in image.mk.

* Make geode subtarget inherit features from the main target instead of
  redefining them.

* For subtargets create device definitions with basic packages set.

Signed-off-by: Tomasz Maciej Nowak <tomek_n@o2.pl>
[rebased]
Signed-off-by: Paul Spooren <mail@aparcar.org>