Plankton: Support DFU mode
Plankton: Support DFU mode

This includes:
  - Enabling SBU path for debug USB usage in USB mode (not in DP mode)
  - Add a console command to enable DFU mode for lab devices. This
    requires the additional Silego chip on the device.

BRANCH=None
BUG=None
TEST=Hook up a Ryu with the additional Silego chip. Issue 'dfu' command
and see DFU device in 'lsusb' output. Issue the command again and the
device goes away.

Change-Id: I3f942c6abe52784351f2242c9b0b6400e9b2b6fd
Signed-off-by: Vic Yang <5fd92abeee651458f98e2856f4f200ee971cee4d@google.com>
Reviewed-on: https://chromium-review.googlesource.com/247350
Tested-by: Vic Yang <5fd92abeee651458f98e2856f4f200ee971cee4d@chromium.org>
Reviewed-by: Alec Berg <1a4429eeda29095a12e19a73405f80f5b7f37a63@chromium.org>
Commit-Queue: Vic Yang <5fd92abeee651458f98e2856f4f200ee971cee4d@chromium.org>
(cherry picked from commit eb86f21b05aaed4dc3bfda51a2eba3f53ec16590)
Signed-off-by: Alec Berg <1a4429eeda29095a12e19a73405f80f5b7f37a63@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/264941

thehobn/ec,thehobn/ec,thehobn/ec,thehobn/ec