Allow Out of Tree Configuration Files

It is difficult to integrate this repo with an ongoing project because they may have an existing build-system for gateware and bare-metal firmware that they would like to keep.

For our project, we use hdlmake and have some pipelines set up already, our hdl is also structured differently with a hdl top module and a substantially different block design.

For this reason I propose that the build system allows the definition of environment variables to place configurations, gateware build results (xsa, bit), and bif files external to the project. This change would allow the boot-image repo to be used as a submodule in other projects.

Another feature that would be useful would be the addition of default configuration files, for example for the atf-conf. these could work by checking if the relevant file exists locally (or is defined externally) and then use the default if not.

I realise we originally discussed the submodule structure in a meeting some time ago, but I hope that adding this as an optional alternative is ok.