Move setting default BOARD_VNDK_VERSION value

Current BOARD_VNDK_VERSION is being set from config.mk, but this is
before build system loads release config and checks if VNDK is
deprecated or not. Move this to envsetup so it can set up
BOARD_VNDK_VERSION only when VNDK is not deprecated with flag.

Bug: 324976878
Test: mainline release target build succeeded

Change-Id: I490cc431038f5cebc40d03c98d2666eaba906664