Adding Build time flag to specify Cow Version

Adding in build time flag for specifying cow version, in preparation to
add new cow version in libsnapshot

Test: m -> cat misc-info.txt
Bug: 279954153
Change-Id: I148d4e7cdd669496c91213889787172ffd83a5ab
diff --git a/core/Makefile b/core/Makefile
index 8b25e9b..cc73c26 100644
--- a/core/Makefile
+++ b/core/Makefile
@@ -5763,6 +5763,8 @@
     echo "virtual_ab_compression_method=$(PRODUCT_VIRTUAL_AB_COMPRESSION_METHOD)" >> $(1))
   $(if $(filter true,$(PRODUCT_VIRTUAL_AB_OTA_RETROFIT)), \
     echo "virtual_ab_retrofit=true" >> $(1))
+  $(if $(PRODUCT_VIRTUAL_AB_COW_VERSION), \
+    echo "virtual_ab_cow_version=$(PRODUCT_VIRTUAL_AB_COW_VERSION)" >> $(1))
 endef
 
 # By conditionally including the dependency of the target files package on the