Multi-pass environment variable resolution

[bobbrow]

@Gouthamm4G

Copied from #957

The initialized variables don't work inside env when it is referenced right next to the definition. For example,

{
    "env": {
        "envRoot": "apps/tool/buildenv",
        "arm6.include": "${envRoot}/arm6/include",
    },
    "configurations": [
        {
            "name": "Custom",
            "includePath": [
                "${arm6.include}"
            ],
            "browse": {
                "limitSymbolsToIncludedHeaders": false,
                "path": [
                     "${arm6.include}"
                ]
            },
            …
        }
    ],
    "version": 4
}

This doesn't seem to work. That would be really useful

[john-patterson]

I'll take a crack at this.

[sean-mcmanus]

@john-patterson Cool, thanks a lot 👍