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/rock2_defconfig b/configs/rock2_defconfig
index 0a95e6a..242b8d5 100644
--- a/configs/rock2_defconfig
+++ b/configs/rock2_defconfig
@@ -74,6 +74,7 @@
 CONFIG_USB_GADGET_DWC2_OTG=y
 CONFIG_USB_DWC2=y
 CONFIG_USB_STORAGE=y
+CONFIG_USB_FUNCTION_MASS_STORAGE=y
 CONFIG_DM_VIDEO=y
 CONFIG_DISPLAY=y
 CONFIG_VIDEO_ROCKCHIP=y