Add support for SPI and CS mode commands

[funkeleinhorn]

Flashrom and Flashprog recently added support for a command to select the SPI mode between half and full duplex SPI and a command to manually control the CS. This PR adds support for these commands. They allow more applications than just interacting with flashes over serprog as for example programming AVR microcontrollers which can be tried with my fork of avrdude. This PR is based on #13

[neuschaefer]

Hi, thanks for your PR!

It looks good overall, but I left some more specific comments.

I reviewed the top three commits (which are not in the multiple-cs branch).

[neuschaefer]

This is suboptimal performance-wise, because of the lack of chunking.

I gave optimization a try and came up with the follow (untested!), but it's not very concise:

void spi_full_duplex(const pio_spi_inst_t *spi, uint32_t wlen, uint32_t rlen) {
    size_t len = MAX(wlen, rlen);
    uint8_t buffer[128];

    putchar(S_ACK);

    while (wlen || rlen) {
        size_t len = MIN(rlen, wlen);
        size_t chunk_size = MIN(sizeof(buffer), len);
        memset(buffer, 0, chunk_size);

        if (wlen) {
            size_t chunk = MIN(wlen, chunk_size);
            fread(buffer, 1, chunk, stdin);
            wlen -= chunk;
        }

        pio_spi_write8_read8_blocking(spi, buffer, buffer, chunk_size);

        if (rlen) {
            size_t chunk = MIN(rlen, chunk_size);
            fwrite(buffer, 1, chunk, stdout);
            rlen -= chunk;
        }
    }
    fflush(stdout);
}

[funkeleinhorn]

I added this suggested change but was not able to test it yet with hardware.

[funkeleinhorn]

@neuschaefer thanks for your review! I addressed the mentioned points. Please have a look again :)