usb: various hacks related to otg

- start switch to b_host role via sysfs
- control vbus output power
  - can be used to start charging when a charger with ID
    grounded is first connected to the device and then to the
    wall outlet
  - enable/disable pulldown resistors via sysfs,
    can be used to emulate a connection to a usb host,
    useful for switching to b_host when using y-cables
  - script for entering b_host mode

Signed-off-by: Andreas Kemnade <andreas@kemnade.info>