Fix unmount/remove race in cleanup_build_dir

With a slow file system, umount can return 0 and the immediately
following remove can fail with a "Device or resource busy" error.
This happened in DevStack in disk-image-create where unmount_image
is followed by an immediate cleanup_build_dir.

Solution is to apply same logic from bug 1332521 to allow the
remove to retry on failure (up to 5s) in case the umount has not
completed.

Change-Id: I3337e2b4ad0111e77f79dc179439cdfea8ebdeda
Closes-Bug: #1527721

lib/common-functions

@@ -173,7 +173,10 @@ function mount_qcow_image() {
 }
 
 function cleanup_build_dir () {
-    sudo rm -rf $TMP_BUILD_DIR/built
+    if ! timeout 5  sh -c " while ! sudo rm -rf $TMP_BUILD_DIR/built; do sleep 1; done"; then
+        echo "ERROR: unable to cleanly remove $TMP_BUILD_DIR/built"
+        exit 1
+    fi
     sudo rm -rf $TMP_BUILD_DIR/mnt
     if tmpfs_check 0; then
         sudo umount -f $TMP_BUILD_DIR || true