Implement directed vendor snapshot

Vendors can now generate only needed modules by setting the following
Makefile variables:

- DIRECTED_VENDOR_SNAPSHOT: set to true
- VENDOR_SNAPSHOT_MODULES: list of snapshot candidates

e.g.

DIRECTED_VENDOR_SNAPSHOT := true
VENDOR_SNAPSHOT_MODULES := toybox_vendor sh_vendor libbase libcutils ...

Bug: 157967325
Test: m dist vendor-snapshot after setting those in BoardConfig.mk
Change-Id: Iea1ddbe78e143316fb6cb5027de90b9c83252f80
diff --git a/core/soong_config.mk b/core/soong_config.mk
index 1d252b7..428984b 100644
--- a/core/soong_config.mk
+++ b/core/soong_config.mk
@@ -144,6 +144,12 @@
 $(call add_json_bool, VndkUseCoreVariant,                $(TARGET_VNDK_USE_CORE_VARIANT))
 $(call add_json_bool, VndkSnapshotBuildArtifacts,        $(VNDK_SNAPSHOT_BUILD_ARTIFACTS))
 
+$(call add_json_bool, DirectedVendorSnapshot,            $(DIRECTED_VENDOR_SNAPSHOT))
+$(call add_json_map,  VendorSnapshotModules)
+$(foreach module,$(VENDOR_SNAPSHOT_MODULES),\
+  $(call add_json_bool,$(module),true))
+$(call end_json_map)
+
 $(call add_json_bool, Treble_linker_namespaces,          $(filter true,$(PRODUCT_TREBLE_LINKER_NAMESPACES)))
 $(call add_json_bool, Enforce_vintf_manifest,            $(filter true,$(PRODUCT_ENFORCE_VINTF_MANIFEST)))