Move board config variable from mainline_system

This variable setting belongs in a board config, not
a product makefile.

Test: presubmit
Change-Id: I41e342ce841ec7aea69ff62aaefdc748cbf28d04
diff --git a/target/board/BoardConfigMainlineCommon.mk b/target/board/BoardConfigMainlineCommon.mk
index be014bf..9bc7e0c 100644
--- a/target/board/BoardConfigMainlineCommon.mk
+++ b/target/board/BoardConfigMainlineCommon.mk
@@ -39,6 +39,9 @@
 # Enable system property split for Treble
 BOARD_PROPERTY_OVERRIDES_SPLIT_ENABLED := true
 
+# Include stats logging code in LMKD
+TARGET_LMKD_STATS_LOG := true
+
 # Generate an APEX image for experiment b/119800099.
 DEXPREOPT_GENERATE_APEX_IMAGE := true