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've added it when I was adding the scripts inside the image feature, with that I'm passing information whether scripts (required and optional) were actually downloaded. This used to be private field, but since I've moved the whole types to separate package I had to make it public.