Add PRODUCT_BOOT_JARS_EXTRA interface

This interface allows vendor to append their boot jars after AOSP's
without changing inherit order.

Test: get_build_var PRODUCT_BOOT_JARS
Change-Id: I6beb8cc9d7fcf0d474b24f634e81657aa42e7ed7
Bug: 152267233
diff --git a/core/product.mk b/core/product.mk
index 364f0d8..b497abb 100644
--- a/core/product.mk
+++ b/core/product.mk
@@ -216,6 +216,12 @@
 
 # A list of module names of BOOTCLASSPATH (jar files)
 _product_list_vars += PRODUCT_BOOT_JARS
+
+# A list of extra BOOTCLASSPATH jars (to be appended after common jars).
+# Products that include device-specific makefiles before AOSP makefiles should use this
+# instead of PRODUCT_BOOT_JARS, so that device-specific jars go after common jars.
+_product_list_vars += PRODUCT_BOOT_JARS_EXTRA
+
 _product_list_vars += PRODUCT_SUPPORTS_BOOT_SIGNER
 _product_list_vars += PRODUCT_SUPPORTS_VBOOT
 _product_list_vars += PRODUCT_SUPPORTS_VERITY