Remove external/ from overlay list

Bug: 111521896

Change-Id: I90b12edcce0cbc53cc8b94bbac664779485c2b6c
diff --git a/lk_inc_aosp.mk b/lk_inc_aosp.mk
index 6490c17..9412bb6 100644
--- a/lk_inc_aosp.mk
+++ b/lk_inc_aosp.mk
@@ -20,7 +20,6 @@
           external/headers \
 	  lk/nxp \
           app \
-          external \
           lib \
           device/arm/generic-arm64 \
           device/arm/vexpress-a15 \