I have a custom udeb for a debian derivative that installs supplementary
debian base systems and configures them as virtual machines or chroots.
However I want to install those in /target/srv/<something> and not in
/target directly.

Due to this I can't easily reuse base-installer's library.sh but I have to
provide my own copy of most functions. It would be nice if library.sh
would respect the $TARGET environment variable.

Please find attached a patch that should implement this.