Do not append VNDK version if VNDK is not used

If BOARD_VNDK_VERSION is not defined, ro.vndk.version will not be
set. In this case, the device, is not able to use versioned
configuration files.
Check BOARD_VNDK_VERSION to append the VNDK version to the namespace
configuration files.

Bug: 71730104
Test: Check boot on both 2016 and 2017 Pixel devices.
Change-Id: I1df27c75de42a526768a156387f22a934cda6d72