This increases write rates (of gifio from #5490) from about 2.4fps to over
5fps by making more efficient use of the SD card protocol.
Because of details of oofatfs, it usually manages 64 writes in a single
CMD25, then two writes in a different area of the SD card (presumably,
filesystem metadata). I couldn't find where to increase "64" to a higher
number. This may depend on the allocation size of the filesystem.
I tried preallocating too, but oddly it significantly lowered the write
rate.
Any trailing data is committed when the file is close()d, or when
the `sync` method of the SDCard object is called.
The RP2040 is new microcontroller from Raspberry Pi that features
two Cortex M0s and eight PIO state machines that are good for
crunching lots of data. It has 264k RAM and a built in UF2
bootloader too.
Datasheet: https://pico.raspberrypi.org/files/rp2040_datasheet.pdf
Testing performed: That a card is successfully mounted on Pygamer with
the built in SD card slot
This module is enabled for most FULL_BUILD boards, but is disabled for
samd21 ("M0"), litex, and pca10100 for various reasons.
