Compile in Omaha support for IoT variants.

Use "PRODUCT_IOT := true" in the product config to enable Omaha support
in update_engine.

Bug: 32549448
Test: `make dist` on an IoT product. The right interface is compiled.

(cherry picked from commit 332b2f1268ec2b98d7ffb3d083791f928177a693)

Change-Id: I6e64dcab215bd9cb73880fb8469cf9343784e48b
diff --git a/Android.mk b/Android.mk
index f6064df..95f99bb 100644
--- a/Android.mk
+++ b/Android.mk
@@ -27,10 +27,12 @@
 # the system layer.
 local_use_libcros := $(if $(BRILLO_USE_LIBCROS),$(BRILLO_USE_LIBCROS),0)
 local_use_mtd := $(if $(BRILLO_USE_MTD),$(BRILLO_USE_MTD),0)
-local_use_omaha := $(if $(BRILLO_USE_OMAHA),$(BRILLO_USE_OMAHA),0)
 local_use_shill := $(if $(BRILLO_USE_SHILL),$(BRILLO_USE_SHILL),0)
 local_use_weave := $(if $(BRILLO_USE_WEAVE),$(BRILLO_USE_WEAVE),0)
 
+# IoT devices use Omaha for updates.
+local_use_omaha := $(if $(filter true,$(PRODUCT_IOT)),1,0)
+
 ifeq ($(local_use_shill),1)
 ifneq ($(local_use_dbus),1)
 $(error USE_SHILL depends on USE_DBUS.)