chromebook: Add rev5 to fit image creation

Add the rev5 snow board to the fit image, and remove spring, since it needs it's own nv-uboot and kernel.
hp-chromebook: Add initial script for building "spring" images.

Spring is the codename for the HP Chromebook.  The only differences between it and the Samsung ARM Chromebook are a different dtb file for the fit image, and a different nv_uboot image.