net: reduce USB network driver config options.

USB network drivers are already handled in drivers/net/usb/Kconfig.
Let's save the maintenance burden of dependencies in drivers/net/Makefile.

The newly introduced USB_NET_DRIVERS umbrella config option defaults
to 'y' so as to minimize the changes of behavior.

Signed-off-by: Francois Romieu <romieu@fr.zoreil.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/Makefile

@@ -61,14 +61,7 @@ obj-$(CONFIG_VMXNET3) += vmxnet3/
 obj-$(CONFIG_XEN_NETDEV_FRONTEND) += xen-netfront.o
 obj-$(CONFIG_XEN_NETDEV_BACKEND) += xen-netback/
 
-obj-$(CONFIG_USB_CATC)          += usb/
-obj-$(CONFIG_USB_KAWETH)        += usb/
-obj-$(CONFIG_USB_PEGASUS)       += usb/
-obj-$(CONFIG_USB_RTL8150)       += usb/
-obj-$(CONFIG_USB_HSO)		+= usb/
-obj-$(CONFIG_USB_USBNET)        += usb/
-obj-$(CONFIG_USB_IPHETH)        += usb/
-obj-$(CONFIG_USB_CDC_PHONET)   += usb/
+obj-$(CONFIG_USB_NET_DRIVERS) += usb/
 
 obj-$(CONFIG_HYPERV_NET) += hyperv/
 obj-$(CONFIG_NTB_NETDEV) += ntb_netdev.o