functions: fix unmounting for local builds

livecd-rootfs creates non-private mounts. When building locally using
the auto/build script unmounting fails.

To unmount dev/pts it is insufficient to make the mount private. Its
parents must be private too. Change teardown_mountpoint() accordingly.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>

live-build/functions

@@ -158,10 +158,17 @@ teardown_mountpoint() {
 
     # ensure we have exactly one trailing slash, and escape all slashes for awk
     mountpoint_match=$(echo "$mountpoint" | sed -e's,/$,,; s,/,\\/,g;')'\/'
-    # sort -r ensures that deeper mountpoints are unmounted first
+
+    # Make all submounts private
     for submount in $(awk </proc/self/mounts "\$2 ~ /$mountpoint_match/ \
                       { print \$2 }" | LC_ALL=C sort -r); do
         mount --make-private $submount
+    done
+
+    # Now we can unmount them
+    # sort -r ensures that deeper mountpoints are unmounted first
+    for submount in $(awk </proc/self/mounts "\$2 ~ /$mountpoint_match/ \
+                      { print \$2 }" | LC_ALL=C sort -r); do
         umount $submount
     done
     recover_sourceslist "${mountpoint}"