Although I would be wary of just rm-ing /turnkey/fab unless you are sure that there are no appliances in a partially built state. It can sometimes cause dramas. Perhaps what might be better would be to clean the products sub directories prior to removal. There are many different ways that could be done, but here is one that should be relatively obvious what it's doing and reasonably safe:

cd /turnkey/fab/products
clean_dir () {
    cd $DIR
    make clean
    ERR="$?"
    cd ..
#   [ "$ERR" == "0" ] && rm -r $DIR
}
for DIR in */; do clean_dir; done

Obviously I commented out the rm line to make sure no one just copy pastes it somewhere without understanding what it will do (it will remove all sub-directories in which "make clean" completes successfully).

Alternatively, you could adjust the tkldev-setup script to clean everything up for you properly without removing anything. E.g. run make clean on each products repo, commit any uncommitted changes; check out the master branch and re-pull from TurnKey (or where-ever the master repo is for appliances not yet in the library)...