servod: expose usb hid enter key

This CL adds the control usb_keyboard_enter_key.
Now by doing
dut-control usb_keyboard_enter_key:tab
the user can trigger a usb keyboard press, and thus a USB HID event
through servod's usb keyboard emulation.
Only works on v2 and v4.

BUG=chromium:869335
TEST=manual testing, keyboard handlers are properly initialized

Change-Id: If327c430b16f518f4d8629ed91fbea648291e879
Signed-off-by: Ruben Rodriguez Buchillon <coconutruben@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/1178762
Commit-Ready: ChromeOS CL Exonerator Bot <chromiumos-cl-exonerator@appspot.gserviceaccount.com>
Reviewed-by: Todd Broch <tbroch@chromium.org>
2 files changed