Change how meta-qt5 repo is used with init script

The manifest.xml and manifest_dizzy.xml no longer checkouts the meta-qt5
repo, but the init script expects it to exists next to the meta-b2qt repo.
This will then work with BYOS where both meta layer are available. For
internal use, add manifest_qt5.xml, which checkouts the meta-qt5 repo from
internal playground server.

Change-Id: Id00f18de80ca11ee4813248bc2a84861df265b6f
Reviewed-by: Eirik Aavitsland <eirik.aavitsland@theqtcompany.com>