ARM: imx_v6_v7_defconfig: build in usbnet to support NFS for non-ethernet board

At some boards, it has no ethernet support. As an alternative, we can use
USB Ethernet card to support NFS (u-boot supports it too). It supports
AXIS cards which are used most frequently.
Signed-off-by: NPeter Chen <peter.chen@nxp.com>
Signed-off-by: NShawn Guo <shawnguo@kernel.org>

arch/arm/configs/imx_v6_v7_defconfig

@@ -146,7 +146,7 @@ CONFIG_MICREL_PHY=y
 CONFIG_USB_PEGASUS=m
 CONFIG_USB_RTL8150=m
 CONFIG_USB_RTL8152=m
-CONFIG_USB_USBNET=m
+CONFIG_USB_USBNET=y
 CONFIG_USB_NET_CDC_EEM=m
 CONFIG_BRCMFMAC=m
 CONFIG_WL12XX=m