dumpvars += BOARD_VNDK_VERSION

Dump variable in log to track remaining targets which don't set it.

Bug: 122954981
Test: $ grep VNDK out/soong.log
2019/03/07 09:22:04.235604 build/soong/ui/build/dumpvars.go:109: BOARD_VNDK_VERSION current

Change-Id: I34146c752a4bf0d799374bb93cc0d75b0f1b048c
diff --git a/ui/build/dumpvars.go b/ui/build/dumpvars.go
index d644f5f..7f0b784 100644
--- a/ui/build/dumpvars.go
+++ b/ui/build/dumpvars.go
@@ -204,6 +204,7 @@
 		"BUILD_BROKEN_PHONY_TARGETS",
 
 		// Not used, but useful to be in the soong.log
+		"BOARD_VNDK_VERSION",
 		"BUILD_BROKEN_ANDROIDMK_EXPORTS",
 		"BUILD_BROKEN_DUP_COPY_HEADERS",
 		"BUILD_BROKEN_ENG_DEBUG_TAGS",