This patch allows a given board to configure which pins are used for the
CAN peripherals, in a similar way to all the other bus peripherals (I2C,
UART, SPI). To enable CAN on a board the mpconfigboard.h file should
define (for example):
#define MICROPY_HW_CAN1_TX (pin_B9)
#define MICROPY_HW_CAN1_RX (pin_B8)
#define MICROPY_HW_CAN2_TX (pin_B13)
#define MICROPY_HW_CAN2_RX (pin_B12)
And the board config file should no longer define MICROPY_HW_ENABLE_CAN.
This is to keep the top-level directory clean, to make it clear what is
core and what is a port, and to allow the repository to grow with new ports
in a sustainable way.
