Attempt to be more brutal by delaying more often, instead of always
promptly servicing the usb stack.
This is implemented via using timer6 to do a known number of
microseconds busy delay, and so only works on platforms that have
reached at least core timer functionality, and provide the
rcc_apb1_frequency variable.
NOTE! This will _fail_ on devices using the st_usbfs drivers at present,
but the code _should_ work, and the tests land to verify that the
library fix, fixes the problem. (see subsequent commit)
There's no F1 discovery style board with usb device, so this is for a "generic"
device. The USB portion should be safe, but there's a led used for bootup that
is board specific, and of course the clock source is board specific.
Related, the openocd config file is rather custom to my own setup, but shows
what you need to customize for your test environment.
Further, as the F1 usb core doesn't include support for soft disconnect, use
the very hacky method of dragging the pin low to force reenumeration on reset.
Very very useful for development purposes!
Karl Palsson