SD-Card SDIO/SPI helper lib

[bog-dan-ro]

SD-Card is a must these days for all projects that requires more storage than the space in the flash.

Even though there are a few implementations out there e.g. pico-extrac (incomplete) and ZuluSCSI (GPLv3), it will be great if the SDK itself provides at least basic sdcard operations like init/deinit, read & write over both SDIO and SPI.

[lurch]

Duplicate of #531 ?

[bog-dan-ro]

@lurch more like a subset (e.g. basic sdcard operations doesn't involve any file system).