Use lazy unmount

Other mountpoints also suffer from the "target is busy" problem
on slow systems like Raspberry Pi Zero W.

setup/pi/setup-teslausb

@@ -363,21 +363,21 @@ function create_usb_drive_backing_files () {
     if [ -e /backingfiles/music_disk.bin ] && mount_with_retry /mnt/music
     then
       touch /mnt/music/.metadata_never_index
-      umount /mnt/music
+      umount -l /mnt/music || setup_progress "failed to unmount /mnt/music"
     fi
 
     if [ -e /backingfiles/lightshow_disk.bin ] && mount_with_retry /mnt/lightshow
     then
       mkdir -p /mnt/lightshow/LightShow
       touch /mnt/lightshow/.metadata_never_index
-      umount /mnt/lightshow
+      umount -l /mnt/lightshow || setup_progress "failed to unmount /mnt/lightshow"
     fi
 
     if [ -e /backingfiles/boombox_disk.bin ] && mount_with_retry /mnt/boombox
     then
       mkdir -p /mnt/boombox/Boombox
       touch /mnt/boombox/.metadata_never_index
-      umount /mnt/boombox
+      umount -l /mnt/boombox || setup_progress "failed to unmount /mnt/boombox"
     fi
 
   else