If mkisofs is not available, do not silently try to build and install the sysutils/cdrtools port.

Instead, error out, and allow the user to install it manually.
Create the sha256 signature right after creating the ISO.

There is duplicated logic in two other release scripts which
try to generate sha256 signatures, so we can get rid of
the duplicate logic in those places, in separate cleanup passes.
The other places are:

(1)  The /root/nightly.sh script.

(2)  The build/build_release.sh script.

Compute sha256 signatures for the full image and GUI_Upgrade image right after we build them.

This will allow us to remove the logic for computing these signatures
from the build/build_release.sh script in a future cleanup pass.

Pull the steps for generating the sha256 signature out of the release script and push it deeper into
the build script, closer to where we actually generate the ISO and txz images.

Only write out sha256 signature to disk if computing sha256 signature succeeded.