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: I8c7f6c922465ae5fe08013b0df6f4f6fbcac36ae
Reviewed-on: https://gerrit.automotivelinux.org/gerrit/c/AGL/meta-agl/+/24890
Tested-by: Jenkins Job builder account <agl-jobbuilder@automotivelinux.org>
ci-image-build: Jenkins Job builder account <agl-jobbuilder@automotivelinux.org>
ci-image-boot-test: Jenkins Job builder account <agl-jobbuilder@automotivelinux.org>