stmhal: Implement selector for USB device mode; improve boot up.

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).