rm_old_work.bbclass

do_rm_old_works() {
    subdir="${TMPDIR}/work/${MULTIMACH_TARGET_SYS}/${PN}"
    if [ -d "$subdir" ]; then
        for v in `ls $subdir`; do
            if [ "$subdir/$v" != "${WORKDIR}" ]; then
                echo "Deleting old work dir $v"
                rm -rf $subdir/$v
            fi
        done
    fi
}

addtask rm_old_works before do_configure