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/board_config.mk b/core/board_config.mk
index db60cee..a6aef87 100644
--- a/core/board_config.mk
+++ b/core/board_config.mk
@@ -358,6 +358,13 @@
 endif
 .KATI_READONLY := BUILDING_USERDATA_IMAGE
 
+# Are we building a vbmeta image
+BUILDING_VBMETA_IMAGE := true
+ifeq ($(PRODUCT_BUILD_VBMETA_IMAGE),false)
+  BUILDING_VBMETA_IMAGE :=
+endif
+.KATI_READONLY := BUILDING_VBMETA_IMAGE
+
 ###########################################
 # Now we can substitute with the real value of TARGET_COPY_OUT_VENDOR
 ifeq ($(TARGET_COPY_OUT_VENDOR),$(_vendor_path_placeholder))