Currently the packages for the gnome and weston OS variants are
installed in the image generation step, which means it is (re)done for
every image variant.

Move the gnome/weston recipes to their own ospack job and simply reuse
the resulting ospack for the image step.

As part of this, also rename the ospack, gnome and weston recipes so
they can easily be chosen based on the variant variable, and update the
README accordingly.

Signed-off-by: Nícolas F. R. A. Prado <nfraprado@collabora.com>