Add common usb_conf.h

[mrx23dot]

It would be a lot easier for IDEs if every configuration would be located in one header file,
instead having to manually configure it in the IDE/and build chain and hunt for missing defines.

usb_conf.h

/* select target */
#define STM32F1
#define STM32F103x6
#define STM32F103xB
#define STM32F1xx
#define USBD_STM32F103

/* +whatever usb config */

Similarly to stm32f1xx_hal_conf.h

It should also be effectively included in every file that uses these macros. (possibly via usb.h)
Nice and self contained solution.

[xcvista]

Currenly we are using -D flags on the compiler configuration for that purpose.