Kconfig: gadget: Move CONFIG_USB_FUNCTION_MASS_STORAGE to Kconfig

This commit moves USB_FUNCTION_MASS_STORAGE config to Kconfig.

Signed-off-by: Lukasz Majewski <lukma@denx.de>
Reviewed-by: Jaehoon Chung <jh80.chung@samsung.com>
diff --git a/configs/evb-rk3288_defconfig b/configs/evb-rk3288_defconfig
index 09a8844..5574ff0 100644
--- a/configs/evb-rk3288_defconfig
+++ b/configs/evb-rk3288_defconfig
@@ -72,6 +72,7 @@
 CONFIG_USB_GADGET_VENDOR_NUM=0x2207
 CONFIG_USB_GADGET_PRODUCT_NUM=0x320a
 CONFIG_USB_GADGET_DWC2_OTG=y
+CONFIG_USB_FUNCTION_MASS_STORAGE=y
 CONFIG_DM_VIDEO=y
 CONFIG_DISPLAY=y
 CONFIG_VIDEO_ROCKCHIP=y