Support referencing the stage dir from snapcraft.yaml

[Sergio Schvezov]

Given the use of the after keyword for parts, it is also sometime desired to reference something inside the stage dir; given the execution location and the relocatable fact on building with snapcraft it makes sense to introduce a SNAPCRAFT_STAGE variable that can be used.

Here's an example:

snapcraft.yaml:

```yaml
parts:
  part1:
    plugin: autotools
    source: .
    configflags:
      - --with-swig: $SNAPCRAFT_STAGE/usr/swig
```

[Sergio Schvezov (sergiusens)]

https://github.com/ubuntu-core/snapcraft/pull/270