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 
The tarring was agreed to be acceptable requirement IIRC, as for `/bin/sh`, even `busybox` has it, so it's not a bit requirement, unless you're baking your own scratch image. Besides without it we won't be even able to run the `assemble` script, because we need some sort of shell to execute commands, right? IMHO this is the cleanest possible solution we can have by now. If there'll be a demand to support images without tar, we can then introduce the idea of first doing docker build introducing tar and then doing the sti build.