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 <romain.izard.pro@gmail.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>