Add further options to customize files after flashing

[easimon]

Reopened from separate branch to free up master on my fork

This MR adds options to copy custom files for

• cloud-init network-config
• cmdline.txt
• usercfg.txt for distributions having "modular" config.txt/syscfg.txt/usercfg.txt instead of only config.txt (e.g. Ubuntu)

Another small addition sneaked in: display information on the auto-detected flash card device node before asking if it's the right one (MacOS only).