Add packagegroup-agl-image-weston to ensure that
Weston will be successfully launched and the demo
applications will appear with image agl-demo-platform
on Raspberry Pi 3 as well as on other machines.
Bug-AGL: SPEC-1427
Change-Id: Iae5ddeea612598750de565fecb0e51b79de56d57 Signed-off-by: Leon Anavi <leon.anavi@konsulko.com>