TIM3 is no longer used by USB CDC for triggering outgoing data, so we
can now make it available to the user.
PWM fading on LED(4) is now gone, but will be reinstated in a new way.
Previously, SPI was configured by a board defining MICROPY_HW_ENABLE_SPIx
to 0 or 1. Now, the board should define MICROPY_HW_SPIx_SCK, MISO, MOSI
and NSS. This makes it the same as how I2C is configured.
This is a hack to free up TIM3 so that it can be used by the user.
Instead we use the PVD irq to call the USB VCP polling function, and
trigger it from SysTick (so SysTick itself does not do any processing).
The feature is enabled for pyboard lite only, since it lacks timers.
