Add the possibility to remove non-free packages while starting the live system (and during system install)

While the non-free list of packages to be installed by default is increasing, we should provide a sane way to remove these bits while starting system and during system install.

All of our non-free packages are installed by system-nonfree metapackage, so we may create a script that checks package's dependencies and uninstalls them.
This script should be included in the standard distribution and a nonfree removal option should be added during boot time (live).

The installer should prompt the user if he wants to leave non-free bits installed or not.