USB support for stm32f4 discovery board

Ok, so the bitbox "alive led" interfered with the discovery setup. On the other hand the discovery boards blue led is setup when usb init routine as the code calls USBH_Usr_Init() in usbh_usr.c, an lit by USBH_USR_UserInput() at the end of the device init phase.

I'm also in favor of separating the ST code from bitbox specifics.