Fix a reference to an undefined variable in a script that's set -u.

debian/changelog

@@ -1,3 +1,9 @@
+livecd-rootfs (2.462) UNRELEASED; urgency=medium
+
+  * Fix a reference to an undefined variable in a script that's set -u.
+
+ -- Steve Langasek <steve.langasek@ubuntu.com>  Fri, 29 Sep 2017 15:10:20 -0400
+
 livecd-rootfs (2.461) artful; urgency=medium
 
   * In subiquity, loadkeys to setup Shift+Tab key combination.

live-build/ubuntu-cpc/hooks/033-disk-image-uefi.binary

@@ -62,7 +62,7 @@ install_grub() {
     efi_boot_dir="/boot/efi/EFI/BOOT"
     chroot mountpoint mkdir -p "${efi_boot_dir}"
 
-    if [ "$SUBPROJECT" = minimize ] && [ -n "$partuuid" ]; then
+    if [ "${SUBPROJECT:-}" = minimize ] && [ -n "$partuuid" ]; then
         # FIXME: code duplicated between 032-disk-image.binary
         # and 033-disk-image-uefi.binary.  We want to fix this to not
         # have initramfs-tools installed at all on these images.