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/Makefile b/core/Makefile
index 6ced027..d486c8e 100644
--- a/core/Makefile
+++ b/core/Makefile
@@ -3396,6 +3396,7 @@
   $(hide) rm -rf $(AVB_CHAIN_KEY_DIR)
 endef
 
+ifdef BUILDING_VBMETA_IMAGE
 INSTALLED_VBMETAIMAGE_TARGET := $(BUILT_VBMETAIMAGE_TARGET)
 $(INSTALLED_VBMETAIMAGE_TARGET): PRIVATE_AVB_VBMETA_SIGNING_ARGS := \
     --algorithm $(BOARD_AVB_ALGORITHM) --key $(BOARD_AVB_KEY_PATH)
@@ -3420,6 +3421,7 @@
 .PHONY: vbmetaimage-nodeps
 vbmetaimage-nodeps:
 	$(build-vbmetaimage-target)
+endif # BUILDING_VBMETA_IMAGE
 
 endif # BOARD_AVB_ENABLE