emulator: Enable dex-preoptimization

This enables the Dex pre-optimization for emulator-specific builds.
The idea is to speed up the initial boot sequence of an SDK AVD by
avoiding the costly on-device preopt pass that is performed otherwise
the first time the AVD is launched.

Benchmarking shows reductions in first boot time from 1m30s to 40s
on a 2.4GHz Xeon when emulating the AOSP master ToT.

Change-Id: I67a9ce9791220c202817864c6e83dc8fd9cbc027
diff --git a/target/board/generic/BoardConfig.mk b/target/board/generic/BoardConfig.mk
index f2295a2..3ba648d 100644
--- a/target/board/generic/BoardConfig.mk
+++ b/target/board/generic/BoardConfig.mk
@@ -30,3 +30,9 @@
 
 # Set /system/bin/sh to mksh, not ash, to test the transition.
 TARGET_SHELL := mksh
+
+# Enable dex-preoptimization to speed up the first boot sequence
+# of an SDK AVD. Note that this operation only works on Linux for now
+ifeq ($(HOST_OS),linux)
+WITH_DEXPREOPT := true
+endif
diff --git a/target/board/generic_x86/BoardConfig.mk b/target/board/generic_x86/BoardConfig.mk
index 1bad20d..0206434 100644
--- a/target/board/generic_x86/BoardConfig.mk
+++ b/target/board/generic_x86/BoardConfig.mk
@@ -21,3 +21,8 @@
 # Set /system/bin/sh to mksh, not ash, to test the transition.
 TARGET_SHELL := mksh
 
+# Enable dex-preoptimization to speed up the first boot sequence
+# of an SDK AVD. Note that this operation only works on Linux for now
+ifeq ($(HOST_OS),linux)
+WITH_DEXPREOPT := true
+endif