Create super_empty.img using Soong

The build rule for this .img file will be created in super_image. It
will use the a minimized file returned by `buildMiscInfo` without any
partition information. This file will be disted in target_files.zip

Bug: 390192334
Test: Verified that this file is bit-identical between make and soong
for aosp_cf_x86_64_phone

Change-Id: I9238d67d2ca8739064509a858d5251f2641896e9
diff --git a/android/variable.go b/android/variable.go
index 853d8ef..2134763 100644
--- a/android/variable.go
+++ b/android/variable.go
@@ -654,6 +654,7 @@
 	ProductUseDynamicPartitions       bool                                     `json:",omitempty"`
 	ProductRetrofitDynamicPartitions  bool                                     `json:",omitempty"`
 	ProductBuildSuperPartition        bool                                     `json:",omitempty"`
+	BuildingSuperEmptyImage           bool                                     `json:",omitempty"`
 	BoardSuperPartitionSize           string                                   `json:",omitempty"`
 	BoardSuperPartitionMetadataDevice string                                   `json:",omitempty"`
 	BoardSuperPartitionBlockDevices   []string                                 `json:",omitempty"`