Cherrypick 944311 from master. do not merge.

emulator: Enable dex-preoptimization in emulator-specific builds

This patch re-enables dex preoptimization in emulator-specific
builds. This considerably speeds up the first-boot of SDK AVDs
since the dex optimization pass is performed during the build
instead.

This was enabled in a previous patch, then reverted due to a
bug in the build system that has been fixed since.

This patch doesn't affect any device build.

Change-Id: Ie711fb4e1672993b7f7f4b8c5313a89d0b8734d6
diff --git a/target/board/generic/BoardConfig.mk b/target/board/generic/BoardConfig.mk
index f247bca..229ea9c 100644
--- a/target/board/generic/BoardConfig.mk
+++ b/target/board/generic/BoardConfig.mk
@@ -31,5 +31,11 @@
 # Set /system/bin/sh to ash, not mksh, to make sure we can switch back.
 TARGET_SHELL := ash
 
+# 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
+
 # Build OpenGLES emulation guest and host libraries
 BUILD_EMULATOR_OPENGL := true
diff --git a/target/board/generic_x86/BoardConfig.mk b/target/board/generic_x86/BoardConfig.mk
index 84b28ca..1a1bc80 100644
--- a/target/board/generic_x86/BoardConfig.mk
+++ b/target/board/generic_x86/BoardConfig.mk
@@ -21,5 +21,11 @@
 # Set /system/bin/sh to ash, not mksh, to make sure we can switch back.
 TARGET_SHELL := ash
 
+# 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
+
 # Build OpenGLES emulation host and guest libraries
 BUILD_EMULATOR_OPENGL := true