ARM: imx_v6_v7_defconfig: Build USB_CONFIGFS into kernel

Some imx chips (6sll, 6ulz, 7ulp) don't have ethernet support and only a
limited number of USB controllers. For such cases NXP suggests
configuring the USB controller as an ethernet gadget for network boot
testing.

Set USB_CONFIGFS to be built into the kernel so that we can configure
the ethernet gadget without needing modules.

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