Add a new variable for deprecating VNDK

Add a new flag variable for deprecating VNDK. This variable should be
passed to Soong, so it can change behavior based on the flag.

Bug: 290157355
Test: build succeeded with KEEP_VNDK=false
Change-Id: I3a9d1041411ad24ff42f76506a8f929e364b868d
diff --git a/core/config.mk b/core/config.mk
index cbcdfbb..392e82e 100644
--- a/core/config.mk
+++ b/core/config.mk
@@ -778,6 +778,9 @@
 
 requirements :=
 
+# Set default value of KEEP_VNDK.
+KEEP_VNDK ?= true
+
 # BOARD_PROPERTY_OVERRIDES_SPLIT_ENABLED can be true only if early-mount of
 # partitions is supported. But the early-mount must be supported for full
 # treble products, and so BOARD_PROPERTY_OVERRIDES_SPLIT_ENABLED should be set