Add USE_OMAHA flag.

What we really meant when we use ifdef BRILLO in Android.mk is if use
omaha, so this patch introduce a new use flag for omaha.

Bug: 28752785
Test: mma

Change-Id: I4f3a81d953943dd4cd0907c2aabc10a6cb9d6acc
diff --git a/update_engine.gyp b/update_engine.gyp
index 2f27dd4..7a3be60 100644
--- a/update_engine.gyp
+++ b/update_engine.gyp
@@ -55,6 +55,7 @@
       'USE_HWID_OVERRIDE=<(USE_hwid_override)',
       'USE_LIBCROS=<(USE_libcros)',
       'USE_MTD=<(USE_mtd)',
+      'USE_OMAHA=1',
       'USE_POWER_MANAGEMENT=<(USE_power_management)',
       'USE_WEAVE=<(USE_buffet)',
     ],