Remove warning about update-payload-key redefined.

Android defines a rule for installing the file
/etc/update_engine/update-payload-key.pub.pem for non-Brillo targets in
the build/core/Makefile, which overrides the rule defined here but
generates a warning. This CL restricts the brillo-update-payload-key to
BRILLO targets only, avoiding the double definition.

Bug: None
TEST=`make dist` doesn't show the warning anymore.

Change-Id: I42fa8970767858af12325e65a4ea418f8c1fcc11
diff --git a/Android.mk b/Android.mk
index 892de7b..27f9aec 100644
--- a/Android.mk
+++ b/Android.mk
@@ -633,6 +633,7 @@
 
 # Update payload signing public key.
 # ========================================================
+ifdef BRILLO
 include $(CLEAR_VARS)
 LOCAL_MODULE := brillo-update-payload-key
 LOCAL_MODULE_CLASS := ETC
@@ -641,6 +642,7 @@
 LOCAL_SRC_FILES := update_payload_key/brillo-update-payload-key.pub.pem
 LOCAL_BUILT_MODULE_STEM := update_payload_key/brillo-update-payload-key.pub.pem
 include $(BUILD_PREBUILT)
+endif  # BRILLO
 
 # Brillo update payload generation script
 # ========================================================