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