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>

doc/README.usb

@@ -127,8 +127,14 @@ To enable USB Host Ethernet in U-Boot, your platform must of course
 support USB with CONFIG_CMD_USB enabled and working. You will need to
 add some config settings to your board header file:
 
+#define CONFIG_CMD_USB		/* the 'usb' interactive command */
 #define CONFIG_USB_HOST_ETHER	/* Enable USB Ethernet adapters */
-#define CONFIG_USB_ETHER_ASIX	/* Asix, or whatever driver(s) you want */
+
+and one or more of the following for individual adapter hardware:
+
+#define CONFIG_USB_ETHER_ASIX
+#define CONFIG_USB_ETHER_MCS7830
+#define CONFIG_USB_ETHER_SMSC95XX
 
 As with built-in networking, you will also want to enable some network
 commands, for example:
@@ -148,7 +154,10 @@ settings should start you off:
 
 You can also set the default IP address of your board and the server
 as well as the default file to load when a 'bootp' command is issued.
-All of these can be obtained from the bootp server if not set.
+However note that encoding these individual network settings into a
+common exectuable is discouraged, as it leads to potential conflicts,
+and all the parameters can either get stored in the board's external
+environment, or get obtained from the bootp server if not set.
 
 #define CONFIG_IPADDR		10.0.0.2  (replace with your value)
 #define CONFIG_SERVERIP		10.0.0.1  (replace with your value)