Support for dexopt postprocessing in merge_target_files.

When using the VSDK, dexopt is not applied during the vendor build.
To avoid a first-boot time regression, dexopt is applied during the
merge stage, by running dexopt on the vendor apps and rebuilding
the vendor image.

Bug: 188179859
Test: Tested in keystone with VSDK target
Change-Id: Ie8e2d0a82850a2901fa6f250433bcbb43f0a97f2
diff --git a/core/Makefile b/core/Makefile
index b44d1d7..a7df61f 100644
--- a/core/Makefile
+++ b/core/Makefile
@@ -4621,6 +4621,12 @@
 ifneq ($(BOARD_PARTIAL_OTA_UPDATE_PARTITIONS_LIST),)
 	$(hide) echo "partial_ota_update_partitions_list=$(BOARD_PARTIAL_OTA_UPDATE_PARTITIONS_LIST)" >> $@
 endif
+ifeq ($(BUILDING_WITH_VSDK),true)
+	$(hide) echo "building_with_vsdk=true" >> $@
+endif
+ifeq ($(TARGET_FLATTEN_APEX),false)
+	$(hide) echo "target_flatten_apex=false" >> $@
+endif
 
 .PHONY: misc_info
 misc_info: $(INSTALLED_MISC_INFO_TARGET)