Install: Small improvements (Fix #162, Close #215)

Created by: ollieparanoid

• #162 (closed): When flasher flash_system runs, do not try to generate the system image anymore, when it does not exist. That failed previously, because the --add parameter was not available. Instead of simply adding it, it throws a meaningful error now. Because otherwise --add would do the expected thing only when the system image is missing - otherwise it would be ignored.
• #215 (closed): Write explicitly which passwords the user will be configuring
• Added information about pmbootstrap export after the installation.