Compile update_engine without shill.

Add a new BRILLO_USE_SHILL for shill, it depends on BRILLO_USE_DBUS.

Bug: 30746265
Test: mma with USE_SHILL=1 and USE_DBUS=1
Test: mma with USE_SHILL=0 and USE_DBUS=1
Test: mma with USE_SHILL=0 and USE_DBUS=0

Change-Id: Ia5ef7a7c7cf48e81c2dd0e4326eeac0f7c0248b3
diff --git a/update_engine.gyp b/update_engine.gyp
index b892672..9e8d735 100644
--- a/update_engine.gyp
+++ b/update_engine.gyp
@@ -56,6 +56,7 @@
       'USE_LIBCROS=<(USE_libcros)',
       'USE_MTD=<(USE_mtd)',
       'USE_OMAHA=1',
+      'USE_SHILL=1',
       'USE_WEAVE=<(USE_buffet)',
     ],
     'include_dirs': [