Send SPI attach command to enable the SPI flash pins

        ESP8266 ROM does this when you send flash_begin, ESP32 ROM
        has it as a SPI command.

def flash_spi_attach(self, hspi_arg):
        """Send SPI attach command to enable the SPI flash pins

        ESP8266 ROM does this when you send flash_begin, ESP32 ROM
        has it as a SPI command.
        """
        # last 3 bytes in ESP_SPI_ATTACH argument are reserved values
        arg = struct.pack('<I', hspi_arg)
        if not self.IS_STUB:
            # ESP32 ROM loader takes additional 'is legacy' arg, which is not
            # currently supported in the stub loader or esptool.py (as it's not usually needed.)
            is_legacy = 0
            arg += struct.pack('BBBB', is_legacy, 0, 0, 0)
        self.check_command("configure SPI flash pins", ESP32ROM.ESP_SPI_ATTACH, arg)