Fix f1 platform

[duckylotl]

This PR contains 3 fixes regarding the F1 platform.
Additionally the hal_conf_extra.h files of the examples got a few extra notes about optional configuration defines.

Alternate pinmap broken

With recent changes the handling of alternate function settings was was changed, letting the Arduino core do the low level work.
This caused a bug where the F1 platform alternate function re-mapping did not work.
A missing define prevented the actual remapping being called.

[...]
#if defined(AFIO_MAPR_CAN_REMAP1)
    case AFIO_CAN1_1:
      __HAL_AFIO_REMAP_CAN1_1();
      break;
    case AFIO_CAN1_2:
      __HAL_AFIO_REMAP_CAN1_2();
      break;
    case AFIO_CAN1_3:
      __HAL_AFIO_REMAP_CAN1_3();
      break;
#endif
[...]

The necessary define is now defined in hal_conf_extra.h by default.
Possible fix for #47, #51, #52, parts of #49

RX pinmode clobbered

The CAN driver does enable the Pullup on the RX pin. The implementation did assume all defined pinmodes by Arduino use the STM_MODE_AF_PP for the CAN pins. This is not the case for the F1 platform, it uses STM_MODE_INPUT for.

This PR contains a fix that no longer touches the mode, instead just modifies the pullup section of the pin function.

Alternate pinmode not defined

F103 platform files don't define a valid AFIO mode for the default pinmap.

WEAK const PinMap PinMap_CAN_TD[] = {
  {PA_12, CAN1, STM_PIN_DATA(STM_MODE_AF_PP, GPIO_NOPULL, AFIO_NONE)},
  {PB_9,  CAN1, STM_PIN_DATA(STM_MODE_AF_PP, GPIO_NOPULL, AFIO_CAN1_2)},
  {NC,    NP,   0}
};

Instead of defining AFIO_CAN1_1 just AFIO_NONE is used. This is no problem if the default is used since it is already active, or changing to another mode since it is defined. But when returning to the default mode after using an alternate mode previously (without reset), the correct mode would not be set, since none is defined (rare case).

This PR contains a workaround that checks for AFIO_NONE and overwrites it with the expected AFIO_CAN1_1 to correctly re-set the AFIO mode.

[pazi88]

Thanks for this fix. I'm not fan of having the define on the hal_conf_extra.h, because you need to also include that in platformio.ini for example. It would be better to have it on the STM32_CAN.h instead. But I'll include this as fix still.