usb: gadget: reword configuration choices

As USB_CONFIGFS is not a part of the "USB Gadget Drivers" choice
anymore, the name for the option and its attached description needs to
be more descriptive. It appears one level higher in the configuration
menu, and without the context provided by the comments for the choice
entry, it needs to make sense on its own.

Conversely, the "USB Gadget Drivers" entry now only introduces the
legacy drivers, where one or more functions are combined in a single
driver. As the configfs option can be used as a full-fledged
alternative, rename the choice entry to show that it is not the only
way to provice service as an USB gadget.

Signed-off-by: Romain Izard <[e-mail]>
Signed-off-by: Felipe Balbi <[e-mail]>

drivers/usb/gadget/Kconfig

···
 # this first set of drivers all depend on bulk-capable hardware.
 
 config USB_CONFIGFS
-	tristate "USB functions configurable through configfs"
+	tristate "USB Gadget functions configurable through configfs"
 	select USB_LIBCOMPOSITE
 	help
 	  A Linux USB "gadget" can be set up through configfs.
···
 	  UAS utilizes the USB 3.0 feature called streams support.
 
 choice
-	tristate "USB Gadget Drivers"
+	tristate "USB Gadget precomposed configurations"
 	default USB_ETH
 	optional
 	help
···
 	  enough of the right types of endpoints, the gadget driver might
 	  not be able work with that controller, or might need to implement
 	  a less common variant of a device class protocol.
+
+	  The available choices each represent a single precomposed USB
+	  gadget configuration. In the device model, each option contains
+	  both the device instantiation as a child for a USB gadget
+	  controller, and the relevant drivers for each function declared
+	  by the device.
 
 source "drivers/usb/gadget/legacy/Kconfig"