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