Revert "Merge master into staging-next"

I merged master into staging-next but accidentally pushed it to master.
This should get us back to 87a19e9048.

This reverts commit ac241fb7a5, reversing
changes made to 76a439239e.

nixos/modules/system/boot/stage-1-init.sh

@@ -334,10 +334,8 @@ mountFS() {
 
     # Filter out x- options, which busybox doesn't do yet.
     local optionsFiltered="$(IFS=,; for i in $options; do if [ "${i:0:2}" != "x-" ]; then echo -n $i,; fi; done)"
-    # Prefix (lower|upper|work)dir with /mnt-root (overlayfs)
-    local optionsPrefixed="$( echo "$optionsFiltered" | sed -E 's#\<(lowerdir|upperdir|workdir)=#\1=/mnt-root#g' )"
 
-    echo "$device /mnt-root$mountPoint $fsType $optionsPrefixed" >> /etc/fstab
+    echo "$device /mnt-root$mountPoint $fsType $optionsFiltered" >> /etc/fstab
 
     checkFS "$device" "$fsType"
 
@@ -356,11 +354,10 @@ mountFS() {
             ;;
     esac
 
-    # Create backing directories for overlayfs
-    if [ "$fsType" = overlay ]; then
-        for i in upper work; do
-             dir="$( echo "$optionsPrefixed" | grep -o "${i}dir=[^,]*" )"
-             mkdir -m 0700 -p "${dir##*=}"
+    # Create backing directories for unionfs-fuse.
+    if [ "$fsType" = unionfs-fuse ]; then
+        for i in $(IFS=:; echo ${options##*,dirs=}); do
+            mkdir -m 0700 -p /mnt-root"${i%=*}"
         done
     fi