To use this feature a port should define MICROPY_HW_SPIFLASH_SIZE_BITS
along with x_CS, x_SCK, x_MOSI, x_MISO (x=MICROPY_HW_SPIFLASH).  This will
then use external SPI flash on those pins instead of the internal flash.

The SPI is done using the software implementation.  There is currently only
support for standard SPI (ie not dual or quad mode).