usb: gadget: f_obex: convert to new function interface with backward compatibility

Converting f_obex to the new function interface requires converting
the f_obex's function code and its users.

This patch converts the f_obex.c to the new function interface.
The file is now compiled into a separate usb_f_obex.ko module.

The old function interface is provided by means of preprocessor
conditional directives. After all users are converted, the old interface
can be removed.

Signed-off-by: Andrzej Pietrasiewicz <andrzej.p@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>
diff --git a/drivers/usb/gadget/Kconfig b/drivers/usb/gadget/Kconfig
index 93217fc..a5f67e1 100644
--- a/drivers/usb/gadget/Kconfig
+++ b/drivers/usb/gadget/Kconfig
@@ -508,6 +508,9 @@
 config USB_F_SERIAL
 	tristate
 
+config USB_F_OBEX
+	tristate
+
 choice
 	tristate "USB Gadget Drivers"
 	default USB_ETH