Add distro-build-manifest per image instead of packagegroup

Add distro-build-manifest per image instead of packagegroup

Adding distro-build-manifest in the packagegroup-agl-core-boot does trigger rebuilds
of the packagegroup if the manifest changes. Avoid this by adding the manifest
directly in the images only (e.g. agl-demo-platform).

This prevents multiple tasks from being triggered and saves time.

Also this recipe is *not* reproducible by design (timestamps, layer hashes).

Bug-AGL: SPEC-3456
Signed-off-by: Jan-Simon Möller <jsmoeller@linuxfoundation.org>
Change-Id: I6845772da1b4ae88506270e6105182f73e00feb2