It seems that the naming of the Icon file provided by Devhelp has changed.
We currently add a desktop link via an overlay, which isn't using the
correct name and hence the wrong icon is being used.

Update the icon name to get the right icon.

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

The sdk-desktop-icon-devhelp overlay still contained the whole contents
of the old sdk overlay before we split it out.

It now only contains the /etc/skel/Desktop/devhelp.desktop file.

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