Split off SD-card driver to separate module

[bessman]

The SD-card driver contains two parts:

• A SPI driver from Microchip Libraries for Applications (MLA): https://github.com/MicrochipTech/mla_fileio/tree/master/drivers/sd_spi
• FatFS: http://elm-chan.org/fsw/ff/00index_e.html

The SD-card driver contains basically zero code written by us (PSLab developers). A few lines of glue, no more. As such, I would prefer to not have it in the main codebase at all, but instead version control it in a separate repo, build it into a library, and pull it in during build with CMake.