usb: gadget: Convert USB_ETHER to Kconfig

The USB Ethernet gadget option has not yet been moved to Kconfig, let's
deal with that.

Reviewed-by: Ɓukasz Majewski <lukma@denx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
diff --git a/configs/am335x_boneblack_vboot_defconfig b/configs/am335x_boneblack_vboot_defconfig
index e8bcb6d..875f0c8 100644
--- a/configs/am335x_boneblack_vboot_defconfig
+++ b/configs/am335x_boneblack_vboot_defconfig
@@ -50,4 +50,5 @@
 CONFIG_G_DNL_MANUFACTURER="Texas Instruments"
 CONFIG_G_DNL_VENDOR_NUM=0x0451
 CONFIG_G_DNL_PRODUCT_NUM=0xd022
+CONFIG_USB_ETHER=y
 CONFIG_LZO=y