buildsystem: allow to set BUILD_ROOT via BUILD_DIR

scripts/clean

@@ -35,7 +35,7 @@ clean_package() {
 }
 
 if [ "${1}" = "--all" -a -n "${2}" ]; then
-  for build_dir in $(ls -1d "${ROOT}/build."*); do
+  for build_dir in $(ls -1d "${BUILD_ROOT}/${BUILD_BASE}."*); do
     load_build_config "${build_dir}" && "${SCRIPTS}/clean" "${2}"
   done
 elif [ -n "${1}" ]; then