Tidy up instructions on verifying live image's integrity/authenticity

[camoz]

• The section Verifying digital signature could just link to Verifying image integrity, and not repeat those instructions. The instructions may have to be adapted a little bit, to make it easy to follow.
• In the (currently repeated) instructions to verify the image integrity, don't mention sha256 from outils, only mention sha256sum from coreutils (which is a dependency of base-system to keep it simple.

[paper42]

• In the (currently repeated) instructions to verify the image integrity, don't mention sha256 from outils, only mention sha256sum from coreutils (which is a dependency of base-system to keep it simple.

I wouldn't mention something so specific, especially when sha256sum is even in the smallest base meta-package.