build-package: unhack the prefix manipulation

The prefix should really be /usr, otherwise the scripts get hardcoded to
the DESTDIR and it's a massive pain to fix them (sed). Stop doing this,
by using DESTDIR=$TARGET_DIR instead.

Recommended by @umlaeute in stackoverflow:
http://stackoverflow.com/questions/14713288/scripts-installed-by-the-deb-package-have-wrong-prefix/14724600