We currently don't provide a default panel configuration. As a result the
user is getting multiple errors at boot, reading "No running instance of
xfce4-panel was found". Depending on the how the user navigates these
messages, the user may find themselves presented with the message
"Modifying the panel is not allowed" on every subsequent boot.

Provide default panel configuration to improve the user experience and to
avoid them being presented with errors.

Signed-off-by: Martyn Welch <martyn.welch@collabora.com>

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>