Bullhead: Set boot-dex2oat-threads

The fallback on boot to the image property has been removed. Set
it explicitly now.

Change-Id: Ied870835d94e9d123eafca859384c3435062c3dd
diff --git a/device.mk b/device.mk
index fde1027..bac85f2 100644
--- a/device.mk
+++ b/device.mk
@@ -381,6 +381,7 @@
 
 # limit dex2oat threads to improve thermals
 PRODUCT_PROPERTY_OVERRIDES += \
+    dalvik.vm.boot-dex2oat-threads=4 \
     dalvik.vm.dex2oat-threads=2 \
     dalvik.vm.image-dex2oat-threads=4