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 
Well, it used to be private, but since I moved `types.go` to its own `api` package and we use it heavily I had to make it public. But I agree with @csrwng that it makes sense moving this an a couple others somewhere down. @csrwng you're open to do it as a follow up :wink: