Fixed issue when tools is just a single table

It does look a bit strange, that it has to reference tools, but that's how it's scoped. Interesting little tidbit, `ipairs` ignores named entries in the table. That's why this code didn't already fail with the `error("bad parameters")`, I did not know that.

tundra --init currently yields a file with this, which won't work if the above patch isn't applied.

    Config {
        Name = "win64-msvc",
        DefaultOnHost = "windows",
        Tools = { "msvc-vs2012"; TargetArch = "x64" }, -- this!
    },