Define the payload key on PRODUCT_IOT builds.

This key needs to be installed in the /etc/update_engine directory.
Update the ifdef to match the new variable.

Bug: 36702887
Test: make clean; make dist # The key is now installed.
Change-Id: Id572239ff852664aded03a74ccda2a773f0f953d
(cherry picked from commit f29fcf055c1455b0194c72c2fd8dcd309205631f)
diff --git a/Android.mk b/Android.mk
index cff7081..16c6381 100644
--- a/Android.mk
+++ b/Android.mk
@@ -1021,7 +1021,7 @@
 
 # Update payload signing public key.
 # ========================================================
-ifdef BRILLO
+ifeq ($(PRODUCT_IOT),true)
 include $(CLEAR_VARS)
 LOCAL_MODULE := brillo-update-payload-key
 LOCAL_MODULE_CLASS := ETC
@@ -1030,7 +1030,7 @@
 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
+endif  # PRODUCT_IOT
 
 # Brillo update payload generation script
 # ========================================================