Add a working CDC TTY USB device

I'm pretty sure it reliably moves data in both directions, but it has a
lot of TODOs left wrt error handling.

Change-Id: I1d96a63eea7bdc08f6ec78c868c4a1ccbfbda600
diff --git a/motors/pistol_grip/BUILD b/motors/pistol_grip/BUILD
index d83b284..364801b 100644
--- a/motors/pistol_grip/BUILD
+++ b/motors/pistol_grip/BUILD
@@ -9,6 +9,7 @@
   deps = [
     '//motors:util',
     '//motors/usb',
+    '//motors/usb:cdc',
     '//motors/core',
   ],
   restricted_to = mcu_cpus,