libpayload: Add usb_generic_(create|remove) functions for unrecognized devices

libpayload: Add usb_generic_(create|remove) functions for unrecognized devices

It might be useful to provide a USB driver in the payload itself instead of in
libpayload. For example there are multiple payloads being built and linked
against the same libpayload, and they might not need or even want to have the
same set of drivers installed.

This change adds two new functions, usb_generic_create and usb_generic_remove,
which behave like the usbdisk_create and usbdisk_remove functions which are
defined for USB mass storage devices. If a USB device isn't recognized and
claimed by one of the built in USB class drivers (currently hub, hid, and msc)
and the create function is defined, then it will be called to give the payload
a chance to use the device. Once it's removed, if usb_generic_remove is
defined it will be called, effectively giving the payload notice.

Built and booted depthcharge on Link. Built depthcharge for Daisy. Built
a netbooting payload, called usb_poll() with those functions implemented, and
verified that they were called and that the devices they were told about were
reasonable and the same as what was reported by lsusb in the booted system.

Change-Id: Ief7c0a513b60849fbf2986ef4ae5c9e7825fef16
Signed-off-by: Gabe Black <gabeblack@google.com>
Reviewed-on: http://review.coreboot.org/2666
Tested-by: build bot (Jenkins)
Reviewed-by: Kimarie Hoot <kimarie.hoot@se-eng.com>
Reviewed-by: Ronald G. Minnich <rminnich@gmail.com>