Ensure that BOARD_VNDK_VERSION is current

BOARD_VNDK_VERSION controls which version of the VNDK is used for the
build. We only support compiling against the current VNDK, and we don't
support installing old prebuilt VNDK libraries yet, so ensure this
variable is set to "current".

Once we support installing old prebuilt VNDK libraries, we'll also need
to disable building modules that use the VNDK.

Test: build-aosp_arm.ninja is the same before/after
Test: Ensure there are no boards that set BOARD_VNDK_VERSION
Test: Set BOARD_VNDK_VERSION := invalid
Change-Id: Ic26c9f44e356a0734f522b4538cb866d5b901d43
(cherry picked from commit 2c3289bf37)

core/binary.mk

@@ -112,12 +112,12 @@ ifneq ($(LOCAL_SDK_VERSION)$(LOCAL_USE_VNDK),)
   # that for the generated libraries. Clip the API level to the minimum where
   # appropriate.
   ifdef LOCAL_USE_VNDK
-    my_ndk_api := $(BOARD_VNDK_VERSION)
+    my_ndk_api := current
   else
     my_ndk_api := $(LOCAL_SDK_VERSION)
-  endif
-  ifneq ($(my_ndk_api),current)
+    ifneq ($(my_ndk_api),current)
       my_ndk_api := $(call math_max,$(my_ndk_api),$(my_min_sdk_version))
+    endif
   endif
 
   my_ndk_api_def := $(my_ndk_api)

core/envsetup.mk

@@ -260,6 +260,13 @@ ifeq ($(AB_OTA_UPDATER),true)
   endif
 endif
 
+# Check BOARD_VNDK_VERSION
+ifdef BOARD_VNDK_VERSION
+  ifneq ($(BOARD_VNDK_VERSION),current)
+    $(error BOARD_VNDK_VERSION: Only "current" is implemented)
+  endif
+endif
+
 # ---------------------------------------------------------------
 # Set up configuration for target machine.
 # The following must be set: