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: Peter Chen <peter.chen@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>