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: NLeonard Crestez <leonard.crestez@nxp.com>
Reviewed-by: NPeter Chen <peter.chen@nxp.com>
Signed-off-by: NShawn Guo <shawnguo@kernel.org>

arch/arm/configs/imx_v6_v7_defconfig

@@ -335,7 +335,7 @@ CONFIG_NOP_USB_XCEIV=y
 CONFIG_USB_MXS_PHY=y
 CONFIG_USB_GADGET=y
 CONFIG_USB_FSL_USB2=y
-CONFIG_USB_CONFIGFS=m
+CONFIG_USB_CONFIGFS=y
 CONFIG_USB_CONFIGFS_SERIAL=y
 CONFIG_USB_CONFIGFS_ACM=y
 CONFIG_USB_CONFIGFS_OBEX=y