This patch adds U-Boot 'usb tree' command functionality to barebox.

Here is an example output:

  1 ID 0000:0000
  |  u-boot EHCI Host Controller
  |
  +-2 ID 05e3:0606
    |   USB2.0 Hub
    |
    +-3 ID 10c4:ea60
    |    Silicon Labs CP2102 USB to UART Bridge Contr P-00-00669
    |
    +-4 ID 05e3:0606
    | |   USB2.0 Hub
    | |
    | +-5 ID 05e3:0608
    | | |   USB2.0 Hub
    | | |
    | | +-6 ID 0d8c:000c
    | |       C-Media USB Headphone Set
    | |
    | +-7 ID 0d8c:000c
    |       C-Media USB Headphone Set
    |
    +-8 ID 0846:1040
         NETGEAR NETGEAR FA120 Adapter

The tree view is enabled with 'usb -t'

Signed-off-by: Antony Pavlov <antonynpavlov@gmail.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>