Card devexp_416 - Support arbitrary images for STI builders

This PR contains:
- [x] Added type for sti script names.
- [x] Removed the need for sti-helper. The untarring is done on the fly when calling assemble script. 
- [x] Added additional parameter that can steer the location of untarred contents.

Still to be done:
- [x] Testing

Nonetheless comments to the build process are welcome since this part is ready.
//cc @bparees @csrwng @smarterclayton 
I think we've missed something here, again.  If we layer the scripts into the image, then we can support even images that do not have tar (you just have to write a static assemble binary that handles tar).

With this implementation, we're back to a model that requires tar (and /bin/sh) be a part of the image (though it does have the advantage of not requiring "assemble" to process an input tar stream, which is nice)