tegra: whistler: reduce and comment network cfg options

CONFIG_CMD_PING/NFS aren't required for Whistler to boot.

Add some comments.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Tom Warren <twarren@nvidia.com>

include/configs/whistler.h

@@ -74,16 +74,14 @@
 #define CONFIG_USB_EHCI_TEGRA
 #define CONFIG_USB_STORAGE
 #define CONFIG_CMD_USB
-#define CONFIG_USB_HOST_ETHER
-#define CONFIG_USB_ETHER_SMSC95XX
-#define CONFIG_USB_ETHER_ASIX
+
+/* USB networking support */
 #define CONFIG_USB_HOST_ETHER
 #define CONFIG_USB_ETHER_SMSC95XX
 #define CONFIG_USB_ETHER_ASIX
 
-#define CONFIG_CMD_NET      /* bootp, tftpboot, rarpboot    */
-#define CONFIG_CMD_NFS      /* NFS support          */
-#define CONFIG_CMD_PING
+/* General networking support */
+#define CONFIG_CMD_NET
 #define CONFIG_CMD_DHCP
 
 #include "tegra2-common-post.h"