basesdk: Fix overlay inclusion by splitting it up

The basesdk overlay was not actually included since `debos` was not
following the symlink to the sdk one.

Avoid the symlink by getting rid for the sdk recipe of the `$arch-$type`
mechanism, which is an ancient relic inherited from the conversion from
the old `cb_build` pipeline.

Splitting the overlay into semantic units also allow us to avoid the
Eclipse icon on the base SDK desktop, where Eclipse is not installed.

Signed-off-by: Emanuele Aina <emanuele.aina@collabora.com>