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/drivers/usb/gadget/Kconfig b/drivers/usb/gadget/Kconfig
index 0d99d84..6825e6b 100644
--- a/drivers/usb/gadget/Kconfig
+++ b/drivers/usb/gadget/Kconfig
@@ -127,6 +127,12 @@
 
 if USB_GADGET_DOWNLOAD
 
+config USB_FUNCTION_MASS_STORAGE
+	bool "Enable USB mass storage gadget"
+	help
+	  Enable mass storage protocol support in U-Boot. It allows exporting
+	  the eMMC/SD card content to HOST PC so it can be mounted.
+
 config USB_FUNCTION_ROCKUSB
         bool "Enable USB rockusb gadget"
         help