boards define BOARD_NAME, so board_name() is not necessary. HW_ARCH was
just a wrapper around board_name().
This patch simplifies to having only one common method px4_board_name().
The mapping of PX4_SPIDEV_ACCEL_MAG to PX4_SPIDEV_ICM is not
a clean approach and the PX4_SPIDEV_MPU is already used by the
mpu9250 leaving the only (and the correct) option to the use
PX4_SPIDEV_ICM and make it map to the reused ACCEL_MAG_CS PC15
as 206080D_CS