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/nokia.c b/drivers/usb/gadget/nokia.c
index f829a80..eebdd0e0 100644
--- a/drivers/usb/gadget/nokia.c
+++ b/drivers/usb/gadget/nokia.c
@@ -37,7 +37,7 @@
  * the runtime footprint, and giving us at least some parts of what
  * a "gcc --combine ... part1.c part2.c part3.c ... " build would.
  */
-#include "f_ecm.c"
+#define USBF_OBEX_INCLUDED
 #include "f_obex.c"
 #include "f_phonet.c"
 #include "u_ether.c"