dm: usb: sandbox: Add an emulator for USB flash devices

This emulator supports USB enumeration and allows a local file to be provided
as the contents of the emulated flash stick. U-Boot can then use the file as
it would a normal device, with all access passing through the usb_stor layer
and the USB stack.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Marek Vasut <marex@denx.de>