Add PRODUCT_BUILD_VBMETA_IMAGE to control vbmeta generation

In line with the existing PRODUCT_BUILD_*_IMAGE variables, add a flag to
control whether vbmeta is generated.

Bug: 139328573
Test: manual
Change-Id: Id71026f9b0de9019e82e8ef594e718a4b0f3628f
diff --git a/core/product.mk b/core/product.mk
index 3d54719..b410e5e 100644
--- a/core/product.mk
+++ b/core/product.mk
@@ -361,6 +361,7 @@
 _product_single_value_vars += PRODUCT_BUILD_USERDATA_IMAGE
 _product_single_value_vars += PRODUCT_BUILD_RECOVERY_IMAGE
 _product_single_value_vars += PRODUCT_BUILD_BOOT_IMAGE
+_product_single_value_vars += PRODUCT_BUILD_VBMETA_IMAGE
 
 _product_list_vars += PRODUCT_UPDATABLE_BOOT_MODULES
 _product_list_vars += PRODUCT_UPDATABLE_BOOT_LOCATIONS