stm32F070 USB

[fenugrec]

Not sure if I'll have time to make a PR. Leaving this as a reminder and to share my findings.
Actually a PR is probably not even the right thing to do anyway, since there is not really a devkit or popular hardware with an F070 ?

The F070 is a bit special in not having USB clock recovery , unlike the F072.

I essentially took examples/stm32/f4/stm32f4-discovery/usb_cdcacm/cdcacm.c
and modified main.c thus :

// F070 can't do USB clock recovery , needs an external clock + PLL to derive the 48MHz ! Here using a 8M xtal
	rcc_clock_setup_in_hse_8mhz_out_48mhz();

	rcc_periph_clock_enable(RCC_USB);
	rcc_set_usbclk_source(RCC_PLL);
	rcc_periph_clock_enable(RCC_GPIOA);
//( Do not set USB pins to Alternate functions)

	usbd_dev = usbd_init(&st_usbfs_v2_usb_driver, &dev, &config,
			usb_strings, 3,
			usbd_control_buffer, sizeof(usbd_control_buffer));