Enable support for Multi-Layered rootfs when overlayfs is in use

This script last worked with Ubuntu 14.04 trusty where aufs was the default.
After that, Ubuntu changed to overlayfs which didn't support multiple layers in
the early versions. Since ??? (at least Ubuntu 15.10 wily), overlayfs now
supports multiple lowerdir's separated by colons. This patch re-enables multiple
squashfs layers.

Tested with casper 1.365

Documentation of overlayfs:
 * https://www.kernel.org/doc/Documentation/filesystems/overlayfs.txt

Signed-off-by: Daniel Alder <daald@users.noreply.github.com>
--- chroot/usr/share/initramfs-tools/scripts/casper_orig	2016-03-23 16:58:39.821410256 +0100
+++ chroot/usr/share/initramfs-tools/scripts/casper	2016-03-23 17:06:30.237655915 +0100
@@ -519,26 +519,14 @@
             mount -t ${UNIONFS} -o noatime,dirs=/cow/upper=rw:$rofsstring ${UNIONFS} "$rootmnt" || panic "${UNIONFS} mount failed"
             ;;
         overlay|overlayfs)
-            # Mount the layers pairwise from the bottom onto rootmnt,
-            # for the second and later layers rootmnt forms the lower layer.
-            mounts=""
-            for mount in /cow $rofslist
+            rofsstring=""
+            for mount in $rofslist
             do
-                mounts="$mount $mounts"
-            done
-            lower=""
-            for mount in $mounts
-            do
-                if [ "$lower" = "" ]; then
-                    lower="$mount"
-                    continue
-                fi
-                mount -t ${UNIONFS} -o "upperdir=$mount/upper,lowerdir=$lower,workdir=$mount/work" \
-                    "$mount" "$rootmnt" || \
-                  mount -t ${UNIONFS} -o "upperdir=$mount/upper,lowerdir=$lower" \
-                      "$mount" "$rootmnt"
-                lower="$rootmnt"
+                rofsstring="$rofsstring:$mount"
             done
+            rofsstring=${rofsstring#:}
+
+            mount -t ${UNIONFS} -o noatime,lowerdir=$rofsstring,upperdir=/cow/upper,workdir=/cow/work ${UNIONFS} "$rootmnt" || panic "${UNIONFS} mount failed"
             ;;
     esac