scripts: lib: repo: Add INITENV_SHALLOW variable to disable shallow clones
Currently if INITENV_INTERACTIVE is disabled (e. g. in CI builds) we use shallow clones for the meta layers to speed up the build. In some cases this is not useful and we need the full git history for the CI to work on.
Introduce INITENV_SHALLOW to force shallow clones on or off independent from INITENV_INTERACTIVE.