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 
no what we discussed is that if you have tar, you'd be able to write nice clean assemble scripts (that handled tar) and would not require the additional build/layer step.

if you have an image without tar, we would do the additional build/layer step so your image could be supported.

assemble can be written as a static binary so it would not require a shell.  same for run.