Refactor thanks to bilibop

bilibop adds some nice bonus:

if the symlink /dev/bilibop exists, it means the system is running from a writable media (USB, MMC and the like, but not DVD);
if you don’t like ‘bilibop’ but prefer ‘Tails’ or ‘bootdev’, just set up BILIBOP_COMMON_BASENAME to the value of your choice in /etc/bilibop/bilibop.conf

This could be used to refactor and simplify stuff.

Parent Task: #6145

Original created by @tails on 6147 (Redmine)

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