Need to wait for the low-level USB driver to send the data over the USB
in-endpoint before the buffer can be used again.  This patch adds a
check for this.

Packets of 64 bytes length are not send to the host until the following
packet is sent.  Fixed by never sending packets of 64 bytes length.

Can now choose at boot up whether the USB device is CDC+MSC or CDC+HID.
Choice is made by an option in boot.py, with default being CDC+MSC.
HID+MSC is not currently supported, but should be easy to implement.

Boot up now has ability to change the reset mode: hold down USR switch
while booting and LEDs will count from 1 to 7 to indicate the boot mode.
Release USR when correct mode is selected.  Current modes are 1 (normal
boot), 2 (safe mode), 3 (reset FS mode).

Late USB enumeration could clear settings after they had been set.
Now fixed by not clearing some settings on init.

RX was blocking if received characters were not being processed, so
CTRL-C would not be picked up.  Now "fixed" by not blocking, but
instead discarding incoming characters if they overflow the buffer.

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.