usb: net: update README.usb to list all USB ethernet options

- extend the discussion of USB network related config options such that
  all available adapter drivers are listed, and that the 'usb' command
  for the interactive prompt and scripting becomes available
- suggest to *not* put individual IP configuration parameters into the
  exectuable, but instead to put them into external environment or fetch
  them from network

Signed-off-by: Gerhard Sittig <gsi@denx.de>
Acked-by: Simon Glass <sjg@chromium.org>
Acked-by: Marek Vasut <marex@denx.de>