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 can handle `WorkingDir` and `Incremental` as part of a followup PR. I'd also add `ExternalRequiredScripts` and `ExternalOptionalScripts` to this list. I created #100 to track it.