Allow overriding of bacnet-stack repo via ENV variables

[abelino]

Overriding can be done via a mix dep configuration:

{:bacnet,
  github: "redwirelabs/bacnet_ex",
  ref: "aa072af",
  system_env: [
    {"BACNET_GIT_REPOSITORY", "git@github.com:hello/bacnet-stack.git"},
    {"BACNET_GIT_REF", "fffffff"}
  ]
}

[abelino]

Closing. This didn't pan out due to hex/elixir reasons. At compile time, we can provide the ENV overrides and they do seem to be available. However, the ENV vars seem to only be available when the dependency is compiled. Any subsequent top project build will result in the ENV vars not being available and thus not working.