Checksum hash

[rdmark]

To add a step to the script for generating a hash for checksum validation after downloading.

[cefiar]

Rather than adding to build.sh, maybe adding a postrun.sh (which build.sh looks for and runs if needed) that runs the commands for the calculation would be a better way to handle it?
Advantage is that if there are any upstream changes to build.sh, there shouldn't be any real issue with merging them if there are no changes.