Add USE flags to the Android.mk

To help conditionally building parts of update_engine based on
per-product settings, this patch ports the USE flags used in the gyp
to make variables in Android.mk

Change-Id: I4cf5295f812d87c251c32947f0ca6abcc7b10ad6
diff --git a/update_engine.gyp b/update_engine.gyp
index 0749503..f00d620 100644
--- a/update_engine.gyp
+++ b/update_engine.gyp
@@ -23,6 +23,7 @@
       # Defaults if the -DUSE_* flags are not passed to gyp is 0. You can set
       # the default value for the USE flag in the ebuild.
       'USE_hwid_override%': '0',
+      'USE_mtd%': '0',
       'USE_power_management%': '0',
     },
     'cflags': [