As per issue #257, servo is better on TIM5 because TIM2 is connected to
more GPIO.

As part of this, rejig the way TIM3 is initialised, since it's now
shared by USB CDC and the blue LED PWM.

New USB HAL is quite a bit improved over previous one.  Now has better
callbacks and flow control.

REPL over USB CDC now works as before, except for soft-reset (since USB
driver uses malloc...).

Enumerates CDC device over USB, but doesn't transmit/receive yet.