Make flashing info a non-step and include generic information

This fixes #1948 (closed) and demotes the flashing information output to a non-step in cases where it would normally show up as the last step (e.g. 5/5). In addition, when no flashing method is configured, a generic advice and wiki link is printed.

I had a bit of trouble following the step-related code in the file and I don't know how to test all the various branches so I'd appreciate a review. I also decided to remove the default values for the step / steps argument in install_system_image because that seemed a bit brittle to maintain with the main step logic implemented in install (for instance, it took me a moment to realize why it was still printing "x/5" even though I had reduced steps to 4 in install.