USB driver in Chopstx

Full speed device is assumed.  Bus powered system is assumed.
API-wise, self powered system is not supported (yet) by this USB driver.

The driver can be used without Chopstx.  An example can be find in
Gnuk (gnuk/regnual).

The USB driver was originally written for STM32F103, which USB
hardware design is considered not mature.  Modern USB hardware design
allows crystal-less design, and/or comes with internal 5V->3V3
regulator, D+-line pull-up support, and VBUS detection.  STM32F103 has
nothing.

To support self powered system, we need to define a hardware interface
for a board detecting VBUS voltage.  Only after detecting VBUS power,
we can enable USB driver (D+/D- lines).  For self powered system,
driving D+/D- lines by fixed pull-up resistor violates the USB
specification.

With STM32F103, there is a common hardware practice having a
gate/transistor for pull-up D+ line.  If it also supports detecting
VBUS, self powerd system can be supported with the gate/transistor.

Such a gate/transistor can be also used for the board to ask
re-enumeration of USB device.  Asking re-enumeration, it is enough to
have SE0 (Single-Ended Zero) state.  

The USB driver doesn't touch such a gate/transistor.  By gpio_init,
the D+-line should be asserted, USB D+ and D- lines should be drived
to 0 (SE0).  When the USB module is enabled, it goes into J and K
state (from SE0 state).