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/tinker-rk3288_defconfig b/configs/tinker-rk3288_defconfig
index a0df3fd..3acc98d 100644
--- a/configs/tinker-rk3288_defconfig
+++ b/configs/tinker-rk3288_defconfig
@@ -76,6 +76,7 @@
 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_USB_HOST_ETHER=y
 CONFIG_USB_ETHER_ASIX=y
 CONFIG_USB_ETHER_SMSC95XX=y