Test GDP branch with submodules/layers set to track branches instead of commits

Currently most layers/submodule in GDP.git are set to specific commits, usually the relevant branch head commit of the current version of yocto/ivi that the GDP release is based off. During GDP release cycles the heads of release branches of these submodules often change, for bugfixes / security backports etc and are only updated in GDP if somebody manually checks upstream.

It could prove beneficial to have a branch, or something in CI that pulls our submodules from the required release branch head, which would in turn would allow us to see if upstream branch changes break anything, but also alert that the changes are present and mergable to master GDP