custom_image: fix build break

Fix build break caused by
https://android-review.googlesource.com/#/c/305575/.

Bug: 33358681
Test: make custom_images

Change-Id: I1ba7cf61491c8593b20bdac58e984f1928697b8c
diff --git a/core/tasks/tools/build_custom_image.mk b/core/tasks/tools/build_custom_image.mk
index ec491fa..34bbfce 100644
--- a/core/tasks/tools/build_custom_image.mk
+++ b/core/tasks/tools/build_custom_image.mk
@@ -90,6 +90,7 @@
 	$(hide) echo "mount_point=$(PRIVATE_MOUNT_POINT)" >> $(PRIVATE_INTERMEDIATES)/image_info.txt
 	$(hide) echo "fs_type=$(PRIVATE_FILE_SYSTEM_TYPE)" >> $(PRIVATE_INTERMEDIATES)/image_info.txt
 	$(hide) echo "partition_size=$(PRIVATE_PARTITION_SIZE)" >> $(PRIVATE_INTERMEDIATES)/image_info.txt
+	$(hide) echo "ext_mkuserimg=$(MKEXTUSERIMG)" >> $(PRIVATE_INTERMEDIATES)/image_info.txt
 	$(if $(PRIVATE_SELINUX),$(hide) echo "selinux_fc=$(SELINUX_FC)" >> $(PRIVATE_INTERMEDIATES)/image_info.txt)
 	$(if $(PRIVATE_SUPPORT_VERITY),\
 	  $(hide) echo "verity=$(PRIVATE_SUPPORT_VERITY)" >> $(PRIVATE_INTERMEDIATES)/image_info.txt;\