Fix vendor variants generation of vndk

Current routine to check whether BOARD_VNDK_VERSION variant is created
or not is not correct, because only non-VNDK modules are installed in
/vendor. Thus, BOARD_VNDK_VERSION variants should only be created with
non-VNDK modules.

Bug: N/A
Test: m nothing
Change-Id: I2ee7821fce407214ac6be4f4b6d62483c907971f