Fix unbound variable in config/binary and zero UEFI partition

* Zero fill space in UEFI partitions, too
* Set LB_ISO_PREPARER to livecd-rootfs to avoid unbound variable in default string

debian/changelog

@@ -1,3 +1,10 @@
+livecd-rootfs (2.408.33) xenial; urgency=medium
+
+  * Zero fill space in UEFI partitions, too
+  * Set LB_ISO_PREPARER to livecd-rootfs to avoid unbound variable in default string
+
+ -- Robert C Jennings <robert.jennings@canonical.com>  Fri, 08 Jun 2018 09:12:03 -0700
+
 livecd-rootfs (2.408.32) xenial; urgency=medium
 
   * snaps: Fixes for snap pre-seeding (LP: #1775710)

live-build/auto/config

@@ -669,6 +669,7 @@ esac
 lb config noauto \
 	--mode ubuntu \
 	--distribution "$SUITE" \
+	--iso-preparer "livecd-rootfs" \
 	--bootstrap-keyring ubuntu-keyring \
 	--binary-images "$BINARY_IMAGES" \
 	--memtest "$MEMTEST" \

live-build/functions

@@ -161,6 +161,9 @@ umount_disk_image() {
 
     local uefi_dev="/dev/mapper${loop_device///dev/}p15"
     if [ -e "$mountpoint/boot/efi" -a -b "$uefi_dev" ]; then
+        # zero fill free space in UEFI partition
+        cat < /dev/zero > "$mountpoint/boot/efi/bloat_file" 2> /dev/null || true
+        rm "$mountpoint/boot/efi/bloat_file"
         umount --detach-loop "$mountpoint/boot/efi"
     fi