Updates for Chromebook script and add the HP Chromebook script

These 2 commits do a tiny bit of cleanup of the Chromebook image script, as well as add the rev5 dtb to the FIT image, so no matter which Samsung ARM Chromebook they get, our image will work with it.  

Introducing a script that builds an image for the HP Chromebook (called Spring).  Image built and tested successfully, by myself as well as an additional Kali user with an HP Chromebook.