Medium building

In the cli, when -S--source is passed, no chroot is used for building unless
explicitly requested via the -C/--chroot option. When no -S--source is passed,
chroot will be used unless the -c/--nochroot option is passed.

In the gui the default source is the medium directory within the installation
dir (as built by the previous stages).
If this is selected chroot commands will be used for the
medium building unless explicitly overridden (uncheck the chroot CheckBox).
If another source is selected chroot commands will only be used if explicitly
requested (check the chroot CheckBox). The chroot dir would be the project's
installation dir (so if this is '/' chroot is never available).