Add support for using FunctionFS in configfs-based USB gadgets.

[ balbi@ti.com : removed redefinition of VERBOSE_DEBUG and few
	trailing whitespaces ]
Signed-off-by: NAndrzej Pietrasiewicz <andrzej.p@samsung.com>
Signed-off-by: NKyungmin Park <kyungmin.park@samsung.com>
Acked-by: NMichal Nazarewicz <mina86@mina86.com>
Signed-off-by: NFelipe Balbi <balbi@ti.com>