Generate the default swapfile on first boot

Changes in either livecd-rootfs or ubuntu-image seem to periodically
break the transfer of the pre-allocated swapfile (copying it in such a
fashion that it winds up "with holes" and thus unable to be used as a
swapfile). Rather than fight this, just use a simple systemd service to
generate the swapfile if it doesn't exist (using fallocate to keep
things snappy).

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

@@ -20,11 +20,35 @@ if [ "$IMAGEFORMAT" == "none" ]; then
         # Make the writable partition grow
         echo "LABEL=writable    /     ext4    defaults,x-systemd.growfs    0 0" >>/etc/fstab
 
-        # Create a 1GB swapfile
-        dd if=/dev/zero of=/swapfile bs=1G count=1
-        chmod 0600 /swapfile
-        mkswap /swapfile
 
-        echo "/swapfile         none  swap    sw    0 0" >>/etc/fstab
+        # Add units for a 1GiB swapfile, generated on first boot
+        cat << EOF > /lib/systemd/system/mkswap.service
+[Unit]
+Description=Create the default swapfile
+DefaultDependencies=no
+Requires=local-fs.target
+After=local-fs.target
+Before=swapfile.swap
+ConditionPathExists=!/swapfile
+
+[Service]
+Type=oneshot
+ExecStartPre=fallocate -l 1GiB /swapfile
+ExecStartPre=chmod 600 /swapfile
+ExecStart=mkswap /swapfile
+
+[Install]
+WantedBy=swap.target
+EOF
+        cat << EOF > /lib/systemd/system/swapfile.swap
+[Unit]
+Description=The default swapfile
+
+[Swap]
+What=/swapfile
+EOF
+        mkdir -p /lib/systemd/system/swap.target.wants
+        ln -s ../mkswap.service /lib/systemd/system/swap.target.wants/
+        ln -s ../swapfile.swap /lib/systemd/system/swap.target.wants/
     fi
 fi