Export BuildingSystemOtherImage to soong

For building the system other image in soong-only builds.

Bug: 390269431
Test: m --soong-only
Change-Id: I14c092d2353cebd7b683047df5aff0b58b2b678a
diff --git a/core/soong_config.mk b/core/soong_config.mk
index de9306b..0345a74 100644
--- a/core/soong_config.mk
+++ b/core/soong_config.mk
@@ -458,6 +458,8 @@
   $(call add_json_list, InternalBootconfig, $(INTERNAL_BOOTCONFIG))
   $(call add_json_str, InternalBootconfigFile, $(INTERNAL_BOOTCONFIG_FILE))
 
+  $(call add_json_bool, BuildingSystemOtherImage, $(BUILDING_SYSTEM_OTHER_IMAGE))
+
   # super image stuff
   $(call add_json_bool, ProductUseDynamicPartitions, $(filter true,$(PRODUCT_USE_DYNAMIC_PARTITIONS)))
   $(call add_json_bool, ProductRetrofitDynamicPartitions, $(filter true,$(PRODUCT_RETROFIT_DYNAMIC_PARTITIONS)))