Tidy up fstab mods on preinstalled desktop images

The UNCONFIGURED FSTAB warning was being left in the result, the discard
option wasn't included, and the fsck flag was 0 (all in marked contrast
to the preinstalled server images).

live-build/ubuntu/hooks/099-ubuntu-image-customization.chroot

@@ -17,9 +17,40 @@ if [ "$IMAGEFORMAT" == "none" ]; then
         /usr/sbin/oem-config-prepare --quiet
         touch "/var/lib/oem-config/run"
 
-        # Make the writable partition grow
-        echo "LABEL=writable    /     ext4    defaults,x-systemd.growfs    0 0" >>/etc/fstab
+        # Update the fstab to include the "discard" option, and
+        # "x-systemd.growfs" to ensure the root partition is expanded on first
+        # boot
+        awk \
+            -v root_fs_label="writable" \
+            -v root_fs_options="discard,x-systemd.growfs" \
+            '
+        BEGIN { OFS="\t"; count=0; }
 
+        # Omit the "UNCONFIGURED" warning if it is still present
+        /^# UNCONFIGURED FSTAB/ { next; }
+
+        # Only modify the first non-comment line where the second field is the
+        # root and omit multiple root definitions
+        /^[^#]/ && $2 == "/" {
+            if (!count) {
+                $1="LABEL=" root_fs_label;
+                $4=root_fs_options;
+                $6="1";
+            }
+            count++;
+            if (count > 1) next;
+        }
+
+        { print; }
+
+        # If we reach the end without seeing a root mount line, add one
+        END {
+            if (!count) {
+                print "LABEL=" root_fs_label, "/", "ext4", root_fs_options, "0", "1";
+            }
+        }
+        ' /etc/fstab > /etc/fstab.new
+        mv /etc/fstab.new /etc/fstab
 
         # Add units for a 1GiB swapfile, generated on first boot
         cat << EOF > /lib/systemd/system/mkswap.service