Adds two new gadget-side utility functions, to support a declarative
style of managing usb configuration descriptors.  The functions fill
buffers from null-terminated vectors of usb descriptors, which are
simple to build or update.

The "ethernet" gadget driver currently has the most interesting config
descriptors.  This uses those functions to replace some complex code with
simpler static declarations; result, it's cleaner.  (And it'll be easier
to add RNDIS configurations later, too.)

Memory savings (or cost, depending on config) was less than 50 bytes;
nothing worth worrying about.