Use flattened APEX packages in ART Buildbot targets.

This is to simplify the use of APEX packages on the ART Buildbot (and
later on Golem).

Test: ART chroot-based on-device testing using the master-art branch
Bug: 121117762
Change-Id: I9973adcbae604d54f4486e713bca2c742ead4526
diff --git a/BoardConfigCommon.mk b/BoardConfigCommon.mk
index 61276d8..f2dfec8 100644
--- a/BoardConfigCommon.mk
+++ b/BoardConfigCommon.mk
@@ -20,3 +20,7 @@
 TARGET_NO_KERNEL := true
 
 TARGET_CPU_SMP := true
+
+# Flatten APEX packages to make them simpler to use in
+# ART testing/benchmarking environments.
+TARGET_FLATTEN_APEX := true