Add a WITH_DEXOPT_BOOT_IMG_ONLY configuration option.

If WITH_DEXOPT_BOOT_IMG_ONLY=true and WITH_DEXPREOPT=true then we will
only preopt the boot.art and boot.oat files, leaving everything else to
be compiled at first boot. This has fast startup times of WITH_DEXPREOPT
but has a smaller space usage and allows one to update the non-image
parts of /system without reflashing.

Bug: 16938924

(cherry picked from commit 440cc769a3617bf99546bdc7599c949e1e858943)

Change-Id: Ib366b6b5ad80f7078f01bf51f9fbc29ea7e5d777
diff --git a/core/product.mk b/core/product.mk
index 89d5982..c9e22eb 100644
--- a/core/product.mk
+++ b/core/product.mk
@@ -262,7 +262,8 @@
 
 _product_stash_var_list += \
 	DEFAULT_SYSTEM_DEV_CERTIFICATE \
-	WITH_DEXPREOPT
+	WITH_DEXPREOPT \
+	WITH_DEXPREOPT_BOOT_IMG_ONLY
 
 #
 # Stash values of the variables in _product_stash_var_list.