Replace SD SPI driver with SDIO

[pelrun]

As noted in #14, mounting the SD card in SPI mode locks it into that mode until a power-cycle, which is problematic for an application that wants to use SDIO mode. Using SDIO mode instead would avoid interfering with whatever the app wants to do.

Not currently a problem for Picocalc, but may be in the future for it or other platforms using uf2loader.

[pelrun]

So this isn't useful for Picocalc, as it's not wired for SDIO. Would still be useful for other platforms like ctag-tbd, but until I have something I can test with, I'll have to leave this one on the backburner.

[ctag-fh-kiel]

Maybe some inspiration from here https://github.com/carlk3/no-OS-FatFS-SD-SDIO-SPI-RPi-Pico
There are some diy boards with SDIO referenced in order to have a hw platform.
Just checked out your last release, great work :)

[ctag-fh-kiel]

This one has sdio sd-card pins for testing, fyi:
https://www.waveshare.com/wiki/RP2040-GEEK
https://files.waveshare.com/wiki/RP2040-GEEK/RP2040-GEEK-Schematic.pdf