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)
As with F103 generic, there's no readily available L1 board with USB device
A custom LED is used to track boot process, but otherwise this should be
portable to any L1 board, except for the openocd configuration file.
Tests pass straight away, which is good, as it uses the existing f103 usb
driver.
