Use += for BOARD_SEPOLICY_*

In our public documentation, recommend += instead of := . This
allows us to have a better inheritance model for policy, specifically
policy located in the vendor directory.

Change-Id: I53ceef298c730d60ab34aa51bde88e9808725ab2
diff --git a/src/devices/tech/security/se-linux.jd b/src/devices/tech/security/se-linux.jd
index 251cafc..e73cc2f 100644
--- a/src/devices/tech/security/se-linux.jd
+++ b/src/devices/tech/security/se-linux.jd
@@ -133,10 +133,10 @@
 </p>
 
 <pre>
-BOARD_SEPOLICY_DIRS := \
+BOARD_SEPOLICY_DIRS += \
         &lt;root&gt;/device/manufacturer/device-name/sepolicy
 
-BOARD_SEPOLICY_UNION := \
+BOARD_SEPOLICY_UNION += \
         genfs_contexts \ 
         file_contexts \ 
         sepolicy.te