stmhal: Add ability to have filesystem stored on external SPI flash.
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).