FILES -------------------------------------------------------------------------
stm32lib/*
 - all the (possibly consolidated) stm32 lib and usb example code
usb.c 
 - USB-specific hardware setup. Interrupts, clocks, etc. handling USB when 
 not "Attached". some low-level callbacks (low power mode, init, reset, 
 resume, etc).
usb_callbacks.c 
 - aka endpoints: handling data transfer when "Configured". calls out to 
 application specific callbacks (i.e. DFU).
usb_descriptor.c 
 - aka application descriptor; big static struct and callbacks for sending
 the descriptor. 
main.c
 - main loop and calling any hardware init stuff. timing hacks for EEPROM 
 writes not to block usb interrupts. logic to handle 2 second timeout then
 jump to user code. 
hardware.c
 - init routines to setup clocks, interrupts, also destructor functions.
 does not include USB stuff. EEPROM read/write functions.
dfu.c
 - mostly the giant FSM case switch, also some USB endpoint callbacks
TODO --------------------------------------------------------------------------
 * pack the structs
 * use sizeof() for usb application descriptor once structs are packed