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/Nintendo_NES_Classic_Edition_defconfig b/configs/Nintendo_NES_Classic_Edition_defconfig
index b9e553e..fbaeb2e 100644
--- a/configs/Nintendo_NES_Classic_Edition_defconfig
+++ b/configs/Nintendo_NES_Classic_Edition_defconfig
@@ -20,3 +20,4 @@
 CONFIG_AXP_ELDO2_VOLT=1800
 CONFIG_USB_MUSB_GADGET=y
 CONFIG_SYS_USB_EVENT_POLL_VIA_INT_QUEUE=y
+CONFIG_USB_FUNCTION_MASS_STORAGE=y