usb/gadget: Kconfig: fix separate building of configfs-enabled functions

USB_CONFGFS_ZZZZ should appear under a tristate option in order to allow
selecting more than one function without building the legacy gadgets.
Now there are two problems:

1) they can't be selected at all, because they depend on USB_CONFIGFS,
and the patch which adds USB_CONFIGFS has not been merged.
2) they don't select USB_LIBCOMPOSITE (which they need but which is
selected by USB_CONFIGFS)

Signed-off-by: Andrzej Pietrasiewicz <andrzej.p@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Acked-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>